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... Autonomous System. The OSPF
protocol is based on SPF or link-state technology. This is a departure
from the Bellman-Ford base used by traditional internet routing
protocols ...
... Autonomous System. This externally derived data is kept separate from
the OSPF protocol's link state data. Each external route can also be
tagged by the advertising router ...
... neighboring routers become adjacent.
Link state advertisement
Describes to the local state of a router ...
... state of the router's interfaces and adjacencies. Each link
state advertisement is flooded throughout the routing domain. The
...
... a Designated Router. The Designated Router generates a link state
advertisement for the multi-access network and has other special
...
... are the X.25 packet and frame levels for PDNs, and the ethernet data
link layer for ethernets.
...
... SPF-based routing protocol. Such protocols are also referred
to in the literature as link-state or distributed-database protocols.
This section gives a brief description of the developments in SPF ...
... modifications dealt with increasing the fault tolerance of the routing
protocol through, among other things, adding a checksum to the link
state advertisements (thereby detecting database corruption). The paper
also included means for reducing the routing ...
... in an
SPF-based protocol. This was accomplished by introducing mechanisms
which enabled the interval between link state advertisements to be
increased by an order of magnitude.
...
... for each broadcast network, which then originates a
link state advertisement for the network.
...
... The topological database (or what has been referred to above as the
directed graph) is pieced together from link state advertisements
generated by the routers. The neighborhood ...
... routers. The neighborhood of each transit vertex is
represented in a single, separate link state advertisement. Figure 4
shows graphically the link state representation of the two kinds of
...
... represented in a single, separate link state advertisement. Figure 4
shows graphically the link state representation of the two kinds of
transit vertices: routers and multi-access ...
...
Figure 3: The resulting directed graph
Figure 4: Individual link state components
__________________________________________
...
... network with three attached routers. The cost of all
links from network N6 to its attached routers is 0. Note that the link
state advertisement ...
... links from network N6 to its attached routers is 0. Note that the link
state advertisement for network N6 is actually generated by one of the
attached routers ...
...
OSPF supports two types of external metrics. Type 1 external metrics
are equivalent to the link state metric. Type 2 external metrics are
greater than the cost of any path internal to the AS. Use of Type 2
...
... 'es is the major cost of
routing a packet, and eliminates the need for conversion of external
costs to internal link state metrics.
Here is an example of Type 1 external metric processing. Suppose that
...
... longer contiguous. In this case the system administrator must restore
backbone connectivity by configuring virtual links.
Virtual links can be configured between any two ...
... connectivity by configuring virtual links.
Virtual links can be configured between any two backbone routers that
...
... have an interface to a common non-backbone area. Virtual links belong
to the backbone. The protocol treats two routers ...
... to the backbone. The protocol treats two routers joined by a virtual
link as if they were connected by an unnumbered point-to-point network.
...
... areas. The topology of the backbone can be enhanced by adding virtual
links. This gives the system administrator some control over the routes
taken by inter-area ...
... router because it belongs to two areas. In order to make the backbone
connected, a virtual link has been configured between routers R10 and
R11.
...
... Note that a failure of the line between routers RT6 and RT10 will cause
the backbone to become disconnected. Configuring another virtual link
between routers RT7 and RT10 will give the backbone ...
... routers RT7 and RT10 will give the backbone more connectivity
and more resistance to such failures. Also, a virtual link between RT7
and RT10 would allow a much shorter path between the third area
(containing N9) and the router ...
... flooding of external advertisements.
There are a couple of restrictions on the use of stub areas. Virtual
links cannot be configured through stub areas. In addition, AS boundary
routers cannot be placed internal to stub areas.
...
... Backbone partitions can be repaired by
configuring virtual links (see Section 15).
Another way to think about area partitions ...
... A router periodically advertises its state, which is also called link
state. Link state is also advertised when a router's state changes ...
... router periodically advertises its state, which is also called link
state. Link state is also advertised when a router's state changes. A
...
... 's state changes. A
router's adjacencies are reflected in the contents of its link state
advertisements. This relationship between adjacencies and link state
allows the protocol to detect dead routers ...
... . A
router's adjacencies are reflected in the contents of its link state
advertisements. This relationship between adjacencies and link state
allows the protocol to detect dead routers in a timely fashion.
...
... database. This database consists of the collection
of link state advertisements received from each router belonging to the
area. From this database ...
... Database Description Summarize database contents
3 Link State Request Database download
4 Link State ...
... uses Hello packets to discover and maintain
neighbor relationships. The Database Description and Link State Request
packets are used in the forming of adjacencies. OSPF's reliable update ...
... Acknowledgment packets.
Each Link State Update packet carries a set of new link state
advertisements one hop further away from their point of origination. A
...
... Each Link State Update packet carries a set of new link state
advertisements one hop further away from their point of origination. A
single Link State Update ...
... Update packet carries a set of new link state
advertisements one hop further away from their point of origination. A
single Link State Update packet may contain the link state
advertisements of several routers ...
... single Link State Update packet may contain the link state
advertisements of several routers. Each advertisement is tagged with
the ID of the originating router ...
... the ID of the originating router and a checksum of its link state
contents. The five different types of OSPF link state ...
... checksum of its link state
contents. The five different types of OSPF link state advertisements
are listed below in Table 9.
...
... ____________________________________________________________________________
1 Router links advs. Originated by all routers. This
advs. advertisement describes the collected
...
... ____________________________________________________________________________
2 Network links Originated for multi-access networks by
...
... ____________________________________________________________________________
____________________________________________________________________________
3,4 Summary link Originated by area border routers, and
advs. flooded throughout their associated
...
... area border routers, and
advs. flooded throughout their associated
area. Each summary link advertisement
describes a route to a destination ...
... AS boundary routers, and
link advs. flooded throughout the AS. Each external
advertisement describes a route ...
... network access
protocols, such as the Ethernet data link layer. Indications must
be passed from from these protocols to OSPF as the network interface ...
... Much of the OSPF functionality is described in terms of its
operation on lists of link state advertisements. For example, the
advertisements that will be retransmitted to an adjacent router
...
... OSPF Hello
packets, Database Description packets and in its link state
advertisements. This enables routers supporting a mix of optional
capabilities to coexist in a single Autonomous System ...
... Database Description packets. A capability mismatch with a neighbor
is this case will result in only a subset of link state advertisements
being exchanged between the two neighbors.
...
... routing table build process can also be affected by the
presence/absence of optional capabilities. For example, since the
optional capabilities are reported in link state advertisements, routers
incapable of certain functions can be avoided when building the shortest
path tree ...
... For this reason the backbone is represented as an area structure.
Virtual links configured
The virtual links configured with this router ...
... router as one endpoint. In
order to have configured virtual links, the router itself must be an
area border router ...
... router itself must be an
area border router. Virtual links are identified by the Router ID
of the other endpoint ...
... endpoint routers must be attached to a common area, called the
virtual link's transit area. Virtual links are part of the
backbone ...
... routers must be attached to a common area, called the
virtual link's transit area. Virtual links are part of the
backbone, and behave as if they were unnumbered point-to-point ...
... networks between the two routers. A virtual link uses the intra-
area routing of its transit area to forward packets. Virtual links ...
... link uses the intra-
area routing of its transit area to forward packets. Virtual links
are brought up and down through the building of the shortest-path
trees ...
...
List of AS external link advertisements
Part of the topological database. These have have originated from
...
... AS boundary router, some of these AS external link
advertisements have been self originated.
...
... router pictured is RT10, from the map in Figure 6. Note
that router RT10 has a virtual link configured to router RT11, with Area
2 as the link ...
... link configured to router RT11, with Area
2 as the link's transit area. This is indicated by the dashed line in
Figure 9. When the virtual link becomes active ...
... 2 as the link's transit area. This is indicated by the dashed line in
Figure 9. When the virtual link becomes active, through the building of
the shortest path tree ...
... backbone than to areas.
The area topological (or link state) database consists of the collection
of router ...
... router links, network links and summary links advertisements that
have originated from the area's routers ...
... links, network links and summary links advertisements that
have originated from the area's routers. This information is flooded
...
... links advertisements
A router links advertisement is generated by each router in the
area. It describes the state ...
... links advertisements
One network links advertisement is generated for each transit
multi-access network ...
... network.
List of summary links advertisements
Summary link advertisements originate from the area's area border
routers ...
... List of summary links advertisements
Summary link advertisements originate from the area's area border
routers. They describe routes to destinations internal to the
...
... route. The backbone
cannot be configured as a stub area. Also, virtual links cannot be
configured through stub areas. For more information, see Section
3.6.
...
... itself is an area border router, then the StubDefaultCost indicates
the cost of the default summary link that the router should
advertise into the area. There can be a separate cost configured
...
... to establish adjacencies over point-to-point networks and virtual links.
The first step in bringing up an adjacency is to synchronize the
neighbors' topological ...
... Database Description packets to its neighbor. Each Database
Description Packet describes a set of link state advertisements
belonging to the database. When the neighbor ...
... belonging to the database. When the neighbor sees a link state
advertisement that is more recent than its own database copy, it makes a
note that this newer advertisement should be requested.
...
... Database Description Packets sent by the master
(polls) are acknowledged by the slave through echoing of the sequence
number. Both polls and their responses contain summaries of link state
data. The master is the only one allowed to retransmit Database
Description Packets. It does so only at fixed intervals, the length of
...
... Database Exchange Process, each router has a list
of those link state advertisements for which the neighbor has more up-
to-date instances. These advertisements are requested in Link State ...
... link state advertisements for which the neighbor has more up-
to-date instances. These advertisements are requested in Link State
Request Packets. Link State Request packets that are not satisfied are
...
... to-date instances. These advertisements are requested in Link State
Request Packets. Link State Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval. When the
Database Description Process has completed and all ...
... Request packets that are not satisfied are
retransmitted at fixed intervals of time RxmtInterval. When the
Database Description Process has completed and all Link State Requests
have been satisfied, the databases are deemed synchronized and the
...
... routers are marked fully adjacent. At this time the adjacency is fully
functional and is advertised in the two routers' link state
advertisements.
The adjacency is used by the flooding ...
... o The Designated Router originates a network links advertisement on
behalf of the network. This advertisement lists the set of routers ...
... Designated Router itself) currently attached to the
network. The Link State ID for this advertisement (see Section
12.1.4) is the IP interface address ...
... routers on the
network. Since the link state databases are synchronized across
adjacencies (through adjacency bring-up and then the flooding ...
... . This will cause the network and all its attached
routers to originate new link state advertisements. Until the
topological databases again converge, some temporary loss of
...
... traffic
lasts only as long as it take to flood the new link state advertisements
(which announce the new Designated Router).
...
... The Backup Designated Router does not generate a network links
advertisement for the network. (If it did, the transition to a new
...
... the flow of routing protocol packets, and in particular Link State
Updates, through the Autonomous System.
...
... This means that all protocol packets travel a single IP hop, except
those sent over virtual links.
All routing protocol ...
... the the address AllSPFRouters. On all other network types (including
virtual links), the majority of OSPF packets are sent as unicasts, i.e.,
...
... destination AllSPFRouters, the Designated Router
and its Backup send both Link State Update Packets and Link State
...
... and its Backup send both Link State Update Packets and Link State
Acknowledgment Packets to the multicast address AllSPFRouters, while all
...
... multicast address AllSPFRouters, while all
other routers send both their Link State Update and Link State
...
... routers send both their Link State Update and Link State
Acknowledgment Packets to the multicast address AllDRouters.
...
... router.[2]
Note that, for most purposes, virtual links act precisely the same as
unnumbered point-to-point networks ...
... unnumbered point-to-point networks. However, each virtual link does
have an interface IP address (discovered during the routing table ...
... IP source when sending packets over the
virtual link.
For more information on the format of specific packet types ...
... Link state update Section 13.3
5 Link state ack Section 13.5
Table 10: Sections describing packet transmission.
...
... the interface it was received on. For routers that have virtual links
configured, it may not be immediately obvious which interface to
...
... to network N8, it may want to associate the packet with the
interface to area 2, or with the virtual link to router RT10 (which is
part of the backbone ...
... part of the backbone). In the following, we assume that the packet is
initially associated with the non-virtual link.[3]
...
... (2) Indicate the backbone. In this case, the packet has been sent
over a virtual link. The receiving router must be an area
border router ...
... source router) must be the other end of a configured virtual
link. The receiving interface must also attach to the virtual
...
... receiving interface must also attach to the virtual
link's configured transit area. If all of these checks succeed,
the packet is accepted and is from now on associated with the
virtual link ...
... link's configured transit area. If all of these checks succeed,
the packet is accepted and is from now on associated with the
virtual link (and the backbone area).
...
... IP header. If the receiving interface is a point-to-point link or a
virtual link, the sender ...
... interface is a point-to-point link or a
virtual link, the sender is identified by the Router ID (source router ...
... Link state update Section 13
5 Link state ack Section 13.7
Table 11: Sections describing packet reception.
...
... adjacencies may develop over an interface. A
router's link state advertisements reflect the state of its interfaces
...
...
InfTransDelay
The estimated number of seconds it takes to transmit a Link State
Update Packet over this interface ...
... Update Packet over this interface. Link state advertisements
contained in the update packet will have their age incremented by
...
... network. The network link state advertisement is labelled with the
Designated Router's IP address ...
... Interface output cost(s)
The cost of sending a packet on the interface, expressed in the link
state metric. This is advertised as the link cost for this
interface ...
... The cost of sending a packet on the interface, expressed in the link
state metric. This is advertised as the link cost for this
interface in the router ...
... interface in the router links advertisement. There may be a
separate cost for each IP Type of Service ...
...
RxmtInterval
The number of seconds between link state advertisement
retransmissions, for adjacencies belonging to this interface ...
... interface. Also
used when retransmitting Database Description and Link State Request
Packets.
...
... To facilitate this, such interfaces are advertised in router links
advertisements as single host routes, whose destination ...
... router must also originate a network
links advertisement for the network node. The advertisement will
...
... network node. The advertisement will
contain links to all routers (including the Designated Router
...
... interface to transition out of Down
state. On virtual links, the interface operational indication is
actually a result of the shortest path calculation (see Section
...
... state changes, it may be necessary to originate a
new router links advertisement. See Section 12.4 for more details.
Some of the required actions below involve generating events for the
...
... Mask). On unnumbered
point-to-point networks and on virtual links this field should be set to
0.
...
... IP multicast address
AllSPFRouters. On virtual links, Hello packets are sent as unicasts
(addressed directly to the other end of the virtual link ...
... virtual links, Hello packets are sent as unicasts
(addressed directly to the other end of the virtual link) every
HelloInterval seconds. On non-broadcast networks ...
... unicasts along this adjacency. Also used in router
links advertisements as the Link ID for the attached network if the
...
... router
links advertisements as the Link ID for the attached network if the
neighboring router ...
... IP address is learned when Hello packets are
received from the neighbor. For virtual links, the neighbor IP
address is learned during the routing table ...
... networks.
The next set of variables are lists of link state advertisements. These
lists describe subsets of the area topological database. There can be
...
... lists describe subsets of the area topological database. There can be
five distinct types of link state advertisements in an area topological
database: router ...
... in an area topological
database: router links, network links, and type 3 and 4 summary links ...
... router links, network links, and type 3 and 4 summary links
(all stored in the area data structure ...
... links, network links, and type 3 and 4 summary links
(all stored in the area data structure), and AS ...
... (all stored in the area data structure), and AS external links (stored
in the global data structure).
...
... in the global data structure).
Link state retransmission list
The list of link state advertisements ...
... Link state retransmission list
The list of link state advertisements that have been flooded but not
acknowledged on this adjacency. These will be retransmitted at
intervals until they are acknowledged, or until the adjacency is
...
...
Database summary list
The complete list of link state advertisements that make up the area
topological database, at the moment the neighbor ...
... in Database
Description packets.
Link state request list
The list of link state advertisements that need to be received from
...
...
Link state request list
The list of link state advertisements that need to be received from
this neighbor in order to synchronize the two neighbors ...
... Database Description packets are
received, and is then sent to the neighbor in Link State Request
packets. The list is depleted as appropriate Link State Update ...
... neighbor in Link State Request
packets. The list is depleted as appropriate Link State Update
packets are received.
...
... state Full and we say that the two routers are fully adjacent. At this
point the adjacency is listed in link state advertisements.
For a more detailed description of neighbor ...
... In this state the router is describing its entire link state
database by sending Database Description packets to the neighbor.
...
... Database Description Packet is
allowed outstanding at any one time. In this state, Link State
Request Packets may also be sent asking for the neighbor's more
...
... Loading
In this state, Link State Request packets are sent to the neighbor
asking for the more recent advertisements that have been discovered
...
... routers are fully adjacent. These
adjacencies will now appear in router links and network links
...
... Database Description packets. Each router now knows what parts of
its link state database are out of date. For more information on
the generation of this event, consult Section 10.8.
...
...
BadLSReq
A Link State Request has been received for a link state
advertisement not contained in the database. This indicates an
...
... BadLSReq
A Link State Request has been received for a link state
advertisement not contained in the database. This indicates an
error in the synchronization ...
...
Loading Done
Link State Updates have been received for all out-of-date portions
of the database. This is indicated by the Link state ...
... Link State Updates have been received for all out-of-date portions
of the database. This is indicated by the Link state request list
becoming empty after the Database Description Process has completed.
...
... neighbor state may cause a new network links
advertisement to be originated (see Section 12.4).
...
...
Action: The router must list the contents of its entire area link
state database in the neighbor Database summary list. The
...
... network links and summary links contained in the area
structure, along with the AS external links ...
... links contained in the area
structure, along with the AS external links contained in the
global structure. AS external link ...
... links contained in the
global structure. AS external link advertisements are
omitted from a virtual neighbor's Database ...
... Section 3.6). Advertisements whose age is equal to MaxAge
are instead added to the neighbor's Link state
retransmission list. A summary of the Database ...
... state is Loading. Start (or
continue) sending Link State Request packets to the neighbor
(see Section 10.9). These are requests for the neighbor ...
... yet received in the Exchange state). These advertisements
are listed in the Link state request list associated with
the neighbor.
...
... neighbor state transitions to 2-Way. The
Link state retransmission list, Database summary list and
...
... retransmission list, Database summary list and
Link state request list are cleared of link state
advertisements.
...
... Database summary list and
Link state request list are cleared of link state
advertisements.
State ...
... retransmission list, Database summary list and Link state
request list are cleared of link state advertisements. Then
...
... Database summary list and Link state
request list are cleared of link state advertisements. Then
the router increments the sequence number ...
... retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
...
... Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
...
... retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
...
... Database summary list
and Link state request list are cleared of link state
advertisements. Also, the inactivity timer is disabled.
...
... retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements.
...
... Database summary list
and Link state request list are cleared of link state
advertisements.
State ...
... retransmission list, Database summary list
and Link state request list are cleared of link state
advertisements.
...
... Database summary list
and Link state request list are cleared of link state
advertisements.
State ...
... Routers connected by point-to-point networks and virtual links always
become adjacent. On multi-access networks ...
... Network
Mask field should not be checked in Hellos received on unnumbered serial
lines or on virtual links.
The receiving ...
... IP header. If the receiving interface is a point-to-point link or a
virtual link, the source is identified by the Router ID ...
... interface is a point-to-point link or a
virtual link, the source is identified by the Router ID found in the
Hello's OSPF packet ...
... an Hello on a point-to-point network (but not on a virtual
link) set the neighbor structure's Neighbor IP address ...
... accepts a received Database Description Packet as the
next in sequence the packet contents are processed as follows. For each
link state advertisement listed, the advertisement's LS type is checked
for validity. If the LS type is unknown (e.g., not one of the LS types
...
... router looks up the advertisement
in its database to see whether it also has an instance of the link state
advertisement. If it does not, or if the database copy is less recent
(see Section 13.1), the link state advertisement ...
... link state
advertisement. If it does not, or if the database copy is less recent
(see Section 13.1), the link state advertisement is put on the Link
state request list so that it can be requested (immediately or at some
later time) in Link State ...
... database copy is less recent
(see Section 13.1), the link state advertisement is put on the Link
state request list so that it can be requested (immediately or at some
later time) in Link State Request Packets.
...
... link state advertisement is put on the Link
state request list so that it can be requested (immediately or at some
later time) in Link State Request Packets.
When the router ...
... Receiving Link State Request Packets ...
...
This section explains the detailed processing of received Link State
Request packets. Received Link State Request Packets specify a list of
...
... This section explains the detailed processing of received Link State
Request packets. Received Link State Request Packets specify a list of
link state advertisements that the neighbor wishes to receive. ...
...
Request packets. Received Link State Request Packets specify a list of
link state advertisements that the neighbor wishes to receive. Link
state Request Packets should be accepted when the neighbor ...
... Request Packets specify a list of
link state advertisements that the neighbor wishes to receive. Link
state Request Packets should be accepted when the neighbor is in states
Exchange, Loading, or Full. In all other states Link State ...
... Link
state Request Packets should be accepted when the neighbor is in states
Exchange, Loading, or Full. In all other states Link State Request
Packets should be ignored.
...
... Packets should be ignored.
Each link state advertisement specified in the Link State Request packet
should be located in the router ...
...
Each link state advertisement specified in the Link State Request packet
should be located in the router's database ...
... should be located in the router's database, and copied into Link State
Update packets for transmission to the neighbor. These ...
...
Update packets for transmission to the neighbor. These link state
advertisements should NOT be placed on the Link state retransmission
...
... Update packets for transmission to the neighbor. These link state
advertisements should NOT be placed on the Link state retransmission
list for the neighbor ...
... retransmission
list for the neighbor. If a link state advertisement cannot be found in
the database, something has gone wrong with the synchronization ...
... state Exchange the Database Description Packets actually contain
summaries of the link state information contained in the router's
database. Each ...
... information contained in the router's
database. Each link state advertisement in the area's topological
database (at the time the neighbor transitions into Exchange ...
... Sending Link State Request Packets ...
...
In neighbor states Exchange or Loading, the Link state request list
contains a list of those link state advertisements that need to be
...
... neighbor states Exchange or Loading, the Link state request list
contains a list of those link state advertisements that need to be
obtained from the neighbor. To request these advertisements, a router ...
... router
sends the neighbor the beginning of the Link state request list,
packaged in a Link State Request packet.
...
... neighbor the beginning of the Link state request list,
packaged in a Link State Request packet.
When the neighbor ...
...
When the neighbor responds to these requests with the proper Link State
Update packet(s), the Link state request list is truncated and a new
...
... responds to these requests with the proper Link State
Update packet(s), the Link state request list is truncated and a new
Link State Request packet is sent. This process continues until the
link state ...
... Update packet(s), the Link state request list is truncated and a new
Link State Request packet is sent. This process continues until the
link state request list becomes empty. Unsatisfied Link State Requests
...
... Link state request list is truncated and a new
Link State Request packet is sent. This process continues until the
link state request list becomes empty. Unsatisfied Link State Requests
are retransmitted at intervals of RxmtInterval. There should be at most
...
... Link State Request packet is sent. This process continues until the
link state request list becomes empty. Unsatisfied Link State Requests
are retransmitted at intervals of RxmtInterval. There should be at most
one Link State ...
... Link State Requests
are retransmitted at intervals of RxmtInterval. There should be at most
one Link State Request packet outstanding at any one time.
When the Link state ...
... Link State Request packet outstanding at any one time.
When the Link state request list becomes empty, and the neighbor state
...
... state after updating the necessary parts of its database.
This is done by sending Link State Request Packets, and receiving Link
State Update ...
... This is done by sending Link State Request Packets, and receiving Link
State Update Packets in response. Note that, while RT1 has waited until
a complete set of Database Description ...
... a complete set of Database Description Packets has been received (from
RT2) before sending any Link State Request Packets, this need not be the
case. RT1 could have interleaved the sending of Link State ...
... Link State Request Packets, this need not be the
case. RT1 could have interleaved the sending of Link State Request
Packets with the reception of Database Description Packets.
...
... OSPF areas. Such routers
originate summary link advertisements. These routing table
entries are used when calculating the inter-area ...
... Section 16.2). These routing table entries may also be
associated with configured virtual links.
AS boundary ...
... Routers that originate AS external link advertisements. These
routing table entries are used when calculating the AS ...
...
Area
This field indicates the area whose link state information has led
to the routing table entry's collection of paths. This is called
...
... OSPF areas.
These are discovered through the examination of received summary
link advertisements. AS external paths are paths to destinations
...
... AS. These are detected through the examination of
received AS external link advertisements.
Cost
...
...
Cost
The link state cost of the path to the destination. For all paths
except type 2 external paths this describes the entire path's cost.
...
... portion of the path internal to the AS. This cost is calculated as
the sum of the costs of the path's constituent links.
Type 2 cost
...
... with type 2 cost of 10, regardless of the cost of the two paths'
internal components.
Link State Origin
Valid only for intra-area ...
... Valid only for intra-area paths, this field indicates the link state
advertisement (router links or network ...
... intra-area paths, this field indicates the link state
advertisement (router links or network links) that directly
...
... router links or network links) that directly
references the destination. For example, if the destination ...
... network, this is the transit network's network links
advertisement. If the destination is a stub network ...
... network, this is the
router links advertisement for the attached router. The
advertisement is discovered during the shortest-path tree ...
... minimum of the set of costs to its individual components.
There is a virtual link configured between routers RT10 and RT11.
Without this configured virtual link ...
... link configured between routers RT10 and RT11.
Without this configured virtual link, RT11 would be unable to advertise
a route for networks ...
... RT4's routing table would improve (i.e., some of the paths in the
routing table would become shorter) if an additional virtual link were
configured between router RT4 and router ...
... configured between router RT4 and router RT3. The new virtual link
would itself be associated with the first entry for area border router
...
... RT3 in Table 13 (an intra-area path through Area 1). This would yield a
cost of 1 for the virtual link. The routing table entries changes that
would be caused by the addition of this virtual link ...
... link. The routing table entries changes that
would be caused by the addition of this virtual link are shown in Table
14.
...
... Link State Advertisements ...
... Each router in the Autonomous System originates one or more link state
advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
link state advertisements forms the ...
... router in the Autonomous System originates one or more link state
advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
link state advertisements forms the link state or topological ...
... originates one or more link state
advertisements. There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
link state advertisements forms the link state or topological database.
...
... . There are five distinct types of link state
advertisements, which are described in Section 4.3. The collection of
link state advertisements forms the link state or topological database.
Each separate type of advertisement has a separate function. Router ...
... .
Each separate type of advertisement has a separate function. Router
links and network links advertisements describe how an area's routers ...
... routers
and networks are interconnected. Summary link advertisements provide a
way of condensing an area's routing information. AS ...
... inter-area 20 RT3 RT11
Table 14: Changes resulting from an additional virtual link.
...
... The Link State Header ...
... The link state header contains the LS type, Link State ID and
Advertising Router fields. The combination of these three fields
...
... Advertising Router fields. The combination of these three fields
uniquely identifies the link state advertisement.
There may be several instances of an advertisement present in the
...
... checksum and LS age fields. These fields are also
contained in the 20-byte link state header.
...
...
Several of the OSPF packet types list link state advertisements. When
the instance is not important, an advertisement is referred to by its LS
type, Link State ...
... link state advertisements. When
the instance is not important, an advertisement is referred to by its LS
type, Link State ID and Advertising Router (see Link State Request
...
... type, Link State ID and Advertising Router (see Link State Request
Packets). Otherwise, the LS sequence number, LS age and LS checksum
fields ...
... checksum
fields must also be referenced.
A detailed explanation of the fields contained in the link state header
follows.
...
...
This field is the age of the link state advertisement in seconds. It
should be processed as an unsigned 16-bit integer ...
... bit integer. It is set to 0 when
the link state advertisement is originated. It must be incremented by
InfTransDelay on every hop of the flooding procedure. Link state
advertisements ...
... link state advertisement is originated. It must be incremented by
InfTransDelay on every hop of the flooding procedure. Link state
advertisements are also aged as they are held in each router's database.
...
... database.
The age of a link state advertisement is never incremented past MaxAge.
Advertisements having age MaxAge are not used in the routing table
...
... routing table
calculation. When an advertisement's age first reaches MaxAge, it is
reflooded. A link state advertisement of age MaxAge is finally flushed
from the database when it is no longer contained on any neighbor ...
... from the database when it is no longer contained on any neighbor Link
state retransmission lists. This indicates that it has been
acknowledged by all adjacent neighbors ...
... acknowledged by all adjacent neighbors. For more information on the
aging of link state advertisements, consult Section 14.
Ages are examined when a router ...
...
Ages are examined when a router receives two instances of a link state
advertisement, both having identical sequence numbers and checksums. An
...
...
The options field in the link state header indicates which optional
capabilities are associated with the advertisement. OSPF ...
... AS external advertisements. It should be
reset in all router links, network links and summary link ...
... router links, network links and summary link advertisements
associated with a stub area. For all link state advertisements ...
... links, network links and summary link advertisements
associated with a stub area. For all link state advertisements, the
...
... links and summary link advertisements
associated with a stub area. For all link state advertisements, the
setting of the E-bit is for informational purposes only; it does not
...
... bit should be
set in a router links advertisement if and only if the router is capable
of calculating separate routes for each IP ...
... T-bit should always be set in network links advertisements. It should
be set in summary link and AS ...
... network links advertisements. It should
be set in summary link and AS external link advertisements if and only
...
... be set in summary link and AS external link advertisements if and only
if the advertisement describes paths for all TOS values, instead of just
...
... TOS exist, but are equivalent to the TOS 0
path. A link state advertisement's T-bit is examined when calculating
the routing table ...
...
The LS type field dictates the format and function of the link state
advertisement. Advertisements of different types have different names
(e.g., router links ...
... link state
advertisement. Advertisements of different types have different names
(e.g., router links or network links). All advertisement types, except
...
... links). All advertisement types, except
the AS external link advertisements (LS type = 5), are flooded
throughout a single area only. AS external link ...
... link advertisements (LS type = 5), are flooded
throughout a single area only. AS external link advertisements are
flooded throughout the entire Autonomous System, excluding stub areas
...
... __________________________________________________
1 These are the router links
advertisements. They describe the
...
... __________________________________________________
2 These are the network links
advertisements. They describe the set
of routers ...
... Section 12.4.2.
__________________________________________________
3 or 4 These are the summary link
advertisements. They describe
inter-area ...
... __________________________________________________
5 These are the AS external link
advertisements. Originated by AS
boundary routers ...
... Link State ID ...
... domain that is being
described by the advertisement. Depending on the advertisement's LS
type, the Link State ID takes on the values listed in Table 16.
LS Type Link State ...
... Link State ID takes on the values listed in Table 16.
LS Type Link State ID
______________________________________________________________________
1 The originating router ...
... Link State ID.
When the link state advertisement is describing a network, the Link
State ID is either the network ...
... When the link state advertisement is describing a network, the Link
State ID is either the network's IP address (as in type 3 summary link ...
... Link
State ID is either the network's IP address (as in type 3 summary link
advertisements and in AS external link ...
... link
advertisements and in AS external link advertisements) or the network's
IP address is easily derivable from the ...
... advertisements) or the network's
IP address is easily derivable from the Link State ID (note that masking
a network links ...
... Link State ID (note that masking
a network links advertisement's Link State ID with the network's subnet ...
... mask yields the network's IP address). When the link state
advertisement is describing a router, the Link State ID is always the
...
... IP address). When the link state
advertisement is describing a router, the Link State ID is always the
described router's OSPF Router ...
... When an AS external advertisement (LS Type = 5) is describing a default
route, its Link State ID is set to DefaultDestination (0.0.0.0).
...
... OSPF Router ID of the advertisement's
originator. For router links advertisements, this field is identical to
the Link State ID field. Network ...
... router links advertisements, this field is identical to
the Link State ID field. Network link advertisements are originated by
...
... the Link State ID field. Network link advertisements are originated by
the network's Designated Router ...
... the network's Designated Router. Summary link advertisements are
originated by area border routers. Finally, AS ...
... originated by area border routers. Finally, AS external link
advertisements are originated by AS boundary routers ...
... 32-bit integer. It is used to
detect old and duplicate link state advertisements. The space of
sequence numbers is linearly ordered. The larger the sequence number
...
... router uses this sequence number the first time it
originates any link state advertisement. Afterwards, the
advertisement's sequence number is incremented each time the router ...
... ; it is commonly referred to as the Fletcher
checksum. It is documented in Annex C of [RFC994]. The link state
header also contains the length of the advertisement in bytes;
subtracting the size of the age field (two bytes) yields the amount of
...
...
The checksum of a link state advertisement is verified in two cases: a)
when it is received in a Link State Update ...
... checksum of a link state advertisement is verified in two cases: a)
when it is received in a Link State Update Packet and b) at times during
the aging of the link state database ...
... Link State Update Packet and b) at times during
the aging of the link state database. The detection of a checksum
failure leads to separate actions in each case. See Sections 13 and 14
...
... The link state database ...
...
A router has a separate link state database for every area to which it
belongs. The link state database has been referred to elsewhere in the
...
... router has a separate link state database for every area to which it
belongs. The link state database has been referred to elsewhere in the
text as the topological database. All routers ...
... The area database is composed of router links advertisements, network
links advertisements, and summary ...
... links advertisements, network
links advertisements, and summary link advertisements (all listed in the
area data structure ...
... advertisements, network
links advertisements, and summary link advertisements (all listed in the
area data structure). In addition, external routes (AS ...
... database. This lookup function is based on an advertisement's LS
type, Link State ID and Advertising Router.[13] There will be a single
instance ...
... Router.[13] There will be a single
instance (the most up-to-date) of each link state advertisement in the
database. The database lookup function is invoked during the ...
... in the
database. The database lookup function is invoked during the link state
flooding procedure (Section 13) and the routing table calculation
...
... lookup function the router can
determine whether it has itself ever originated a particular link state
advertisement, and if so, with what LS sequence number.
...
... flooding process (Section 13) or b) it is
originated by the router itself (Section 12.4). A link state
advertisement is deleted from a router's database ...
... routing domain (Section 14). Whenever
a link state advertisement is deleted from the database it must also be
...
... it must also be
removed from all neighbors' Link state retransmission lists (see Section
10).
...
... OSPF link state advertisements (with the exception of network links
advertisements) specify metrics. In router links ...
... links
advertisements) specify metrics. In router links advertisements, the
metrics indicate the costs of the described interfaces. In summary link ...
... links advertisements, the
metrics indicate the costs of the described interfaces. In summary link
and AS external link ...
... link
and AS external link advertisements, the metric indicates the cost of
the described path. In all of these advertisements, a separate metric
can be specified for each IP ...
... IP TOS. TOS is encoded in an OSPF link state
advertisement as the following mapping of the Delay (D), Throughput ...
... Originating link state advertisements ...
...
A router may originate many types of link state advertisements. A
router originates a router ...
... . A
router originates a router links advertisement for each area to which it
belongs. If the router is also the Designated Router ...
... .
Area border routers originate a single summary links advertisement for
each known inter-area destination ...
... routers originate a
single AS external links advertisement for each known AS external
destination. ...
... route can be flooded without reflooding the entire
collection of routes. During the flooding procedure, many link state
advertisements can be carried by a single Link State Update packet.
...
... collection of routes. During the flooding procedure, many link state
advertisements can be carried by a single Link State Update packet.
...
... backbone. Router RT4
originates 5 distinct link state advertisements into the backbone (one
router ...
... N1-N4).
Router RT4 will also originate 8 distinct link state advertisements into
Area 1 (one router links ...
... link state advertisements into
Area 1 (one router links and seven summary link advertisements as
pictured in Figure 7). If RT4 has been selected as Designated Router ...
... Area 1 (one router links and seven summary link advertisements as
pictured in Figure 7). If RT4 has been selected as Designated Router
...
... router RT5 will be originating 3 distinct AS
external link advertisements (one for each of the networks N12-N14).
These will be flooded throughout the entire AS ...
...
N12-N14 will not be flooded into area 3 (see Section 3.6). Instead,
router RT11 would originate a default summary link advertisement that
would be flooded throughout area 3 (see Section 12.4.3). This instructs
all of area 3's internal routers ...
... RT11.
Whenever a new instance of a link state advertisement is originated, its
LS sequence number is incremented, its LS age is set to 0, its LS
...
... LS sequence number is incremented, its LS age is set to 0, its LS
checksum is calculated, and the advertisement is added to the link state
database and flooded out the appropriate interfaces. See Section 13.2
for details concerning the installation of the advertisement into the
...
... and flooded out the appropriate interfaces. See Section 13.2
for details concerning the installation of the advertisement into the
link state database. See Section 13.3 for details concerning the
flooding of newly originated advertisements.
...
... flooding of newly originated advertisements.
The eight events that cause a new instance of a link state advertisement
to be originated are:
...
... refresh timer for each
link state advertisement that the router has originated. The LS
refresh ...
... timer guarantees periodic originations regardless of any
other events that cause new instances. This periodic updating of
link state advertisements adds robustness to the link state
algorithm ...
... other events that cause new instances. This periodic updating of
link state advertisements adds robustness to the link state
algorithm. Link state advertisements ...
... link state
algorithm. Link state advertisements that solely describe
unreachable destinations should not be refreshed, but should instead
...
...
When whatever is being described by a link state advertisement changes,
a new advertisement is originated. Two instances of the same link state ...
... When whatever is being described by a link state advertisement changes,
a new advertisement is originated. Two instances of the same link state
advertisement may not be originated within the time period
...
... state changes (see Section 9.1). This may mean that
it is necessary to produce a new instance of the router links
advertisement.
...
... network's Designated Router changes. A new router links
advertisement should be originated. Also, if the router itself is
...
... may mean that it is necessary to produce a new instance of the
router links advertisement. Also, if the router is itself the
Designated Router ...
... Designated Router for the attached network, a new network links
advertisement should be produced.
...
... route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
...
... route has been added/deleted/modified in the routing
table. This may cause a new instance of a summary links
advertisement (for this route) to be originated in each attached
...
... router becomes newly attached to an area. The router must then
originate summary link advertisements into the newly attached area
for all pertinent intra-area and inter-area ...
... EGP) changes. This will cause the AS
boundary router to originate a new instance of an external links
advertisement.
...
... advertisement.
The construction of each type of the link state advertisement is
explained below. In general, these sections describe the contents of
the advertisement body (i.e., the part coming after the 20-byte ...
...
advertisement header). For information concerning the building of the
link state advertisement header, see Section 12.1.
...
... A router originates a router links advertisement for each area that it
belongs to. Such an advertisement describes the collected states of the
router's ...
... advertisement for each area that it
belongs to. Such an advertisement describes the collected states of the
router's links to the area. The advertisement is flooded throughout the
particular area, and no further.
...
...
The format of a router links advertisement is shown in Appendix A
(Section A.4.2). The first 20 bytes of the advertisement consist of the
generic link state ...
... links advertisement is shown in Appendix A
(Section A.4.2). The first 20 bytes of the advertisement consist of the
generic link state header that was discussed in Section 12.1. Router
...
... header that was discussed in Section 12.1. Router
links advertisements have LS type = 1. The router indicates whether it
is willing to calculate separate routes for each IP ...
... router, by setting the appropriate bits in its router links
advertisements. This enables paths to those types of routers to be
...
... routers to be
saved in the routing table, for later processing of summary link
advertisements and AS external link ...
... advertisement then describes the router's working
connections (links) to the area. Each link is typed according to the
...
... 's working
connections (links) to the area. Each link is typed according to the
_________________________________________
...
...
kind of attached network. Each link is also labelled with its Link ID.
This ID gives a name to the entity ...
... kind of attached network. Each link is also labelled with its Link ID.
This ID gives a name to the entity that is on the other end of the link ...
... Link ID.
This ID gives a name to the entity that is on the other end of the link.
Table 18 summarizes the values used for the type and Link ID fields.
...
... entity that is on the other end of the link.
Table 18 summarizes the values used for the type and Link ID fields.
Link type Description ...
... Table 18 summarizes the values used for the type and Link ID fields.
Link type Description Link ID
____________________________________________________________________________
...
... fields.
Link type Description Link ID
____________________________________________________________________________
1 Point-to-point link ...
... Link ID
____________________________________________________________________________
1 Point-to-point link Neighbor Router ID
...
... links advertisement.
In addition, the Link Data field is specified for each link. This field
...
... In addition, the Link Data field is specified for each link. This field
gives 32 bits of extra information for the link ...
... router interface (this is needed by the routing table
calculation, see Section 16.3). For links to stub networks, this field
specifies the network ...
... IP address mask.
Finally, the cost of using the link for output (possibly specifying a
different cost for each type of service) is specified. The output cost
...
... different cost for each type of service) is specified. The output cost
of a link is configurable. It must always be non-zero.
...
... non-zero.
To further describe the process of building the list of link records,
suppose a router wishes to build router ...
... suppose a router wishes to build router links advertisement for an Area
A. The router examines its collection of interface ...
...
o If the attached network does not belong to Area A, no links are
added to the advertisement, and the next interface should be
...
... interface is Point-to-Point, then add
links according to the following:
- If the neighboring router ...
...
- If the neighboring router is fully adjacent, add a Type 1 link
(point-to-point) if this is an interface ...
... link (virtual link) if this is a
virtual link. The Link ID should be set to the Router ID of the
...
... link) if this is a
virtual link. The Link ID should be set to the Router ID of the
neighboring router ...
... Router ID of the
neighboring router, and the Link Data should specify the
interface IP address.
...
... point-to-point network (i.e, not a virtual
link and not an unnumbered point-to-point network) and the
...
... state of the interface is Loopback, add a Type 3 link
(stub network) as long as this is not an interface ...
... network) as long as this is not an interface to an unnumbered
serial line. The Link ID should be set to the IP interface address,
...
... o Else if the state of the interface is Waiting, add a Type 3 link
(stub network) whose Link ID ...
... IP network number of the
attached network and whose Link Data is the attached network's
address ...
... Designated Router and is fully adjacent
to at least one other router, add a single Type 2 link (transit
network) whose whose link ID ...
... Designated Router (which may be the router
itself) and whose Link Data is the interface IP address. Otherwise,
add a link ...
... Link Data is the interface IP address. Otherwise,
add a link as if the interface state were Waiting (see above).
...
... state were Waiting (see above).
Unless otherwise specified, the cost of each link generated by the above
procedure is equal to the output cost of the associated interface. Note
...
... procedure is equal to the output cost of the associated interface. Note
that in the case of serial lines, multiple links may be generated by a
single interface.
...
... After consideration of all the router interfaces, host links are added
to the advertisement by examining the list of attached hosts. A host ...
...
As an example, consider the router links advertisements generated by
router RT3, as pictured in Figure 6. The area containing router RT3
...
... routers have similar addressing schemes. In addition, assume that
all links are functional, and that Router IDs are assigned as the
smallest IP interface ...
...
RT3 originates two router links advertisements, one for Area 1 and one
for the backbone. Assume that router ...
... Designated router for network 192.1.1.0. RT3's router links
advertisement for Area 1 is then shown below. It indicates that RT3 has
two connections ...
... advertisement for Area 1 is then shown below. It indicates that RT3 has
two connections to Area 1, the first a link to the transit network
192.1.1.0 and the second a link ...
... link to the transit network
192.1.1.0 and the second a link to the stub network 192.1.4.0. Note
that the transit network ...
... is identified by the IP interface of its
Designated Router (i.e., the Link ID = 192.1.1.4 which is the Designated
Router RT4's IP interface to 192.1.1.0). Note also that RT3 has
...
... IP address of Designated Router
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network ...
... Link ID = 192.1.4.0 ;IP Network number
Link Data = 0xffffff00 ;Network mask
Type = 3 ;connects to stub network ...
...
Next RT3's router links advertisement for the backbone is shown. It
indicates that RT3 has a single attachment to the backbone ...
... indicates that RT3 has a single attachment to the backbone. This
attachment is via an unnumbered point-to-point link to router RT6. RT3
has again indicated that it is TOS ...
... Section 12.3.
As an example, suppose the point-to-point link between routers RT3 and
RT6 in Figure 15 is a satellite link ...
... point-to-point link between routers RT3 and
RT6 in Figure 15 is a satellite link. The AS administrator may want to
...
... Router RT3
would then originate the following router links advertisement for the
backbone (IP ...
...
A network links advertisement is generated for every transit multi-
access network. (A transit network is a ...
... attached routers). The network links advertisement describes all the
routers that are attached to the network.
...
... router on the network. The network links
advertisement is flooded throughout the area that contains the transit
network, and no further. The ...
... advertisement is flooded throughout the area that contains the transit
network, and no further. The networks links advertisement lists those
routers that are fully adjacent to the Designated Router; each fully
...
... Designated
Router includes itself in this list.
The Link State ID for a network links advertisement is the IP interface ...
... 's
address mask (which is also contained in the network links
advertisement) yields the network's IP address ...
... network, but
is no longer, should flush the network links advertisement that it had
previously originated. This advertisement is no longer used in the
routing table calculation. It is flushed by prematurely incrementing
...
...
As an example of a network links advertisement, again consider the area
configuration in Figure 6. Network links ...
... links advertisement, again consider the area
configuration in Figure 6. Network links advertisements are originated
for network N3 in Area 1, networks ...
... Designated
Router for network N3, the following network links advertisement is
generated by RT4 on behalf of network N3 (see Figure 15 for the address ...
... Summary links ...
... advertisement describes a route to a single
destination. Summary link advertisements are flooded throughout a
single area only. The destination described is one that is external to
...
... Autonomous System.
The DefaultDestination can also be specified in summary link
advertisements. This is used when implementing OSPF's stub area
...
... functionality (see Section 3.6). In a stub area, instead of importing
external routes each area border router originates a "default summary
link" (Link State ID = DefaultDestination) into the area.
...
... external routes each area border router originates a "default summary
link" (Link State ID = DefaultDestination) into the area.
Summary link ...
... Link State ID = DefaultDestination) into the area.
Summary link advertisements are originated by area border routers. The
precise summary routes to advertise into an area are determined by
...
... AS boundary router are
advertised in summary link advertisements. If the routing table
entry's Destination ...
...
o AS external routes are never advertised in summary link
advertisements. If the routing table entry has Path-type type 1
...
...
o Else, if the area associated with this set of paths is the Area A
itself, do not generate a summary link advertisement for the
route.[14 ...
... AS boundary router,
generate a Type 4 link state advertisement for the destination, with
Link State ...
... link state advertisement for the destination, with
Link State ID equal to the AS boundary router's ID and metric equal
...
... route, generate a Type 3 advertisement for the destination, with
Link State ID equal to the network's address and metric equal to the
...
... router's directly
attached areas. In general, this information must be condensed
before appearing in summary link advertisements. Remember that an
area has been defined as a list of address ranges ...
... address,mask] pair. A single Type 3 advertisement
must be made for each range, with Link State ID equal to the range's
address ...
... networks.
If virtual links are being used to provide/increase connectivity of
the backbone, routing information ...
... routing information concerning the backbone networks
should not be condensed before being summarized into the virtual
links' transit areas. In other words, the backbone ranges should be
...
... backbone ranges should be
ignored when originating summary links into these areas. The
existence of virtual links can be determined during the shortest
...
... ignored when originating summary links into these areas. The
existence of virtual links can be determined during the shortest
path calculation for the backbone (see Section 16.1).
...
... router
is an area border router, it should advertise a default summary link
into Area A. The Link State ID for the advertisement should be set to
...
... area border router, it should advertise a default summary link
into Area A. The Link State ID for the advertisement should be set to
DefaultDestination, and the metric set to the (per-area) configurable
parameter StubDefaultCost.
...
... domain.
For an example of summary link advertisements, consider again the area
configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all
...
... configuration in Figure 6. Routers RT3, RT4, RT7, RT10 and RT11 are all
area border routers, and therefore are originating summary links
advertisements. Consider in particular router RT4. Its routing table ...
... RT4. Its routing table
was calculated as the example in Section 11.3. RT4 originates summary
link advertisements into both the backbone and Area 1. Into the
backbone, ...
... networks N9,N10,N11 and host H9
are advertised by a single summary link. This condensation was
originally performed by the router RT11.
...
...
These advertisements are illustrated graphically in Figures 7 and 8.
Two of the summary link advertisements originated by router RT4 follow.
The actual IP addresses ...
... for each Type of Service. All these entries must be considered when
building the summary link advertisement for the destination; a single
advertisement must specify the separate costs (if they exist) for each
...
... Clearing the T-bit in the Options field of a summary link advertisement
indicates that there is a TOS 0 path to the destination ...
...
AS external link advertisements describe routes to destinations external
to the Autonomous System ...
... to the Autonomous System. Most AS external link advertisements describe
routes to specific external destinations. However, a default route ...
... Autonomous System can be described in an AS external advertisement
by setting the advertisement's Link State ID to DefaultDestination
(0.0.0.0). AS external link ...
... Link State ID to DefaultDestination
(0.0.0.0). AS external link advertisements are originated by AS
boundary routers. An AS boundary ...
... router originates a single AS external
link advertisement for each external route that it has learned, either
through another routing protocol ...
...
In general, AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements ...
... In general, AS external link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
...
... link advertisements are the only type of link
state advertisements that are flooded throughout the entire Autonomous
System; all other types of link state advertisements are specific to a
single area. However, AS external advertisements are not flooded
...
... AS external advertisements are not flooded
into/throughout stub areas (see Section 3.6). This enables a reduction
in link state database size for routers internal to stub areas.
...
... The metric that is advertised for an external route can be one of two
types. Type 1 metrics are comparable to the link state metric. Type 2
metrics are assumed to be larger than the cost of any intra-AS path. As
...
... metrics are assumed to be larger than the cost of any intra-AS path. As
with summary link advertisements, if separate paths exist based on TOS,
separate TOS ...
... separate TOS costs can be included in the AS external link
advertisement. The encoding of TOS ...
... advertisement. The encoding of TOS in OSPF link state advertisements is
described in Section 12.3. If the T-bit ...
... If a router advertises an AS external link advertisement for a
destination which then becomes unreachable, the router must then flush
...
...
For an example of AS external link advertisements, consider once again
the AS pictured in Figure 6. There are two AS boundary ...
... routers: RT5 and
RT7. Router RT5 originates three external link advertisements, for
networks N12-N14. Router RT7 originates two external ...
... advertisements, for
networks N12-N14. Router RT7 originates two external link
advertisements, for networks N12 and N15. Assume that RT7 has learned
...
... EGP information with the non-OSPF
router RTX. RTA must then originate AS external link state
advertisements for those destinations it has learned from RTX. By using
the AS ...
... EGP peer RTX.
The resulting AS external link advertisement is pictured below. Note
that the Link State ID is set to DefaultDestination.
...
... AS external link advertisement is pictured below. Note
that the Link State ID is set to DefaultDestination.
; Default route ...
... TOS-capable
LS type = 5 ;indicates AS external link
Link State ID = DefaultDestination ; default route ...
... originated the set of external advertisements, the routing would remain
the same, and the size of the link state database would decrease.
However, it must be unambiguously defined as to which router originates
...
... router having the
lower OSPF Router ID can then flush its advertisement. Flushing a link
state advertisement is discussed in Section 14.1.
...
... Link State Update packets provide the mechanism for flooding link state
advertisements. A Link State Update packet may contain several distinct
...
... Update packets provide the mechanism for flooding link state
advertisements. A Link State Update packet may contain several distinct
advertisements, and floods each advertisement one hop further from its
...
... flooding procedure reliable, each
advertisement must be acknowledged separately. Acknowledgments are
transmitted in Link State Acknowledgment packets. Many separate
acknowledgments can be grouped together into a single packet.
...
... The flooding procedure starts when a Link State Update packet has been
received. Many consistency ...
... before being handed to the flooding procedure (see Section 8.2). In
particular, the Link State Update packet has been associated with a
particular neighbor ...
... All types of link state advertisements, other than AS external links,
are associated with a specific area. However, link state advertisements
...
... AS external links,
are associated with a specific area. However, link state advertisements
do not contain an area field. A link state advertisement's area must be
...
... are associated with a specific area. However, link state advertisements
do not contain an area field. A link state advertisement's area must be
deduced from the Link State Update ...
... do not contain an area field. A link state advertisement's area must be
deduced from the Link State Update packet header.
...
... packet header.
For each link state advertisement contained in the packet, the following
steps are taken:
...
... checksum
turns out to be invalid, discard the advertisement and get the next
one from the Link State Update packet.
...
... Update packet.
(2) Examine the link state advertisement's LS type. If the LS type is
unknown, discard the advertisement and get the next one from the
Link State ...
... link state advertisement's LS type. If the LS type is
unknown, discard the advertisement and get the next one from the
Link State Update Packet. This specification defines LS Types 1-5
(see Section 4.3).
...
... AS external advertisement (LS type = 5), and the
area has been configured as a stub area, discard the advertisement
and get the next one from the Link State Update Packet. AS external
...
... (4) Else if the advertisement's age is equal to MaxAge, and there is
currently no instance of the advertisement in the router's link
state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a Link
State ...
... link
state database, then take the following actions:
(a) Acknowledge the receipt of the advertisement by sending a Link
State Acknowledgment packet back to the sending neighbor (see
Section 13.5).
...
... (b) Purge all outstanding requests for equal or previous instances
of the advertisement from the sending neighbor's Link State
Request list (see Section 10).
...
... state Exchange or in state
Loading, then install the MaxAge advertisement in the link state
database. Otherwise, simply discard the advertisement. In
either case, examine the next advertisement (if any) listed in
the Link State ...
... link state
database. Otherwise, simply discard the advertisement. In
either case, examine the next advertisement (if any) listed in
the Link State Update packet.
...
... (5) Otherwise, find the instance of this advertisement that is currently
contained in the router's link state database. If there is no
database copy, or the received advertisement is more recent than the
...
... was installed less than MinLSInterval seconds ago, discard the
new advertisement (without acknowledging it) and examine the
next advertisement (if any) listed in the Link State Update
packet.
...
... retransmission lists.
(d) Install the new advertisement in the link state database
(replacing the current database copy). This may cause the
...
...
(e) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
interface ...
... interface. This is explained below in Section 13.5.
(f) If this new link state advertisement indicates that it was
originated by this router itself, the router ...
... router itself, the router must advance the
advertisement's link state sequence number, and issue a new
instance of the advertisement (see Section 13.4).
...
... (6) Else, if there is an instance of the advertisement on the sending
neighbor's Link state request list, an error has occurred in the
Database Description process. In this case, restart ...
... neighbor event BadLSReq for
the sending neighbor and stop processing the Link State Update
packet.
...
... steps should be performed:
(a) If the advertisement is listed in the Link state retransmission
list for the receiving ...
... treat the received advertisement as an acknowledgment, by
removing the advertisement from the Link state retransmission
list. This is termed an "implied acknowledgment". Its
...
...
(b) Possibly acknowledge the receipt of the advertisement by sending
a Link State Acknowledgment packet back out the receiving
interface ...
... network management, discard the advertisement and process the next
link state advertisement contained in the packet.
...
... Determining which link state is newer ...
...
When a router encounters two instances of a link state advertisement, it
must determine which is more recent. This occurred above when comparing
a received advertisement to the database ...
... adjacency bring-up.
A link state advertisement is identified by its LS type, Link State ID
and Advertising Router ...
...
A link state advertisement is identified by its LS type, Link State ID
and Advertising Router. For two instances of the same advertisement,
...
... Installing link state advertisements in the database ...
... The best route to the destination described by the summary link
advertisement must be re-examined (see Section 16.5). If this
destination ...
... router, it may also be necessary to
re-examine all the AS external link advertisements.
AS external ...
... route to the destination described by the AS external link
advertisement must be re-examined (see Section 16.6).
...
... interfaces and adding the advertisement to the appropriate neighbors'
Link state retransmission lists. Also included in this part of the
flooding procedure is the maintenance of the ...
... lists. Also included in this part of the
flooding procedure is the maintenance of the neighbors' Link state
request lists.
...
... links (LS Type = 5)
AS external links are flooded throughout the entire AS, with the
exception of stub areas (see Section 3.6). The eligible interfaces ...
... are all the router's interfaces, excluding virtual links and those
interfaces attaching to stub areas.
...
... interfaces attaching to the Area
A. If Area A is the backbone, this includes all the virtual links.
Link state ...
... , this includes all the virtual links.
Link state databases must remain synchronized over all adjacencies
associated with the above eligible interfaces ...
... interface. It should be
noted that this procedure may decide not to flood a link state
advertisement out a particular interface, if there is a high probability
that the attached neighbors ...
... that the neighbors eventually do receive the advertisement, so the
advertisement is still added to each adjacency's Link state
retransmission list. For each eligible interface:
...
... neighbor state is
Exchange or Loading), examine the Link state request list
associated with this adjacency. If there is an instance of the
new advertisement on the list, it indicates that the neighboring
...
... o If the two copies are the same instance, then delete the
advertisement from the Link state request list, and try the
next neighbor.[16 ...
... o Else, the new advertisement is more recent. Delete the
advertisement from the Link state request list.
(c) If the new advertisement was received from this neighbor ...
... neighbor has an
up-to-date instance of this new advertisement. Add the new
advertisement to the Link state retransmission list for the
adjacency. This ensures that the flooding ...
... (2) The router must now decide whether to flood the new link state
advertisement out this interface. If in the previous step, the link
state advertisement was NOT added to any of the Link state ...
... flood the new link state
advertisement out this interface. If in the previous step, the link
state advertisement was NOT added to any of the Link state
retransmission ...
... link state
advertisement out this interface. If in the previous step, the link
state advertisement was NOT added to any of the Link state
retransmission lists, there is no need to flood ...
... (5) If this step is reached, the advertisement must be flooded out the
interface. Send a Link State Update packet (with the new
advertisement as contents) out the interface ...
... multicast.
The destination IP address specified for the Link State Update
Packet depends on the state ...
... non-broadcast, multi-access networks, separate Link State Update
packets must be sent, as unicasts ...
... Receiving self-originated link state ...
...
It is a common occurrence to receive a self-originated link state
advertisement via the flooding procedure. If the advertisement received
is a newer instance than the last instance that the router ...
... router must take special action.
The reception of such an advertisement indicates that there are link
state advertisements in the routing domain that were originated before
...
... sequence number, and originate a new instance of the advertisement.
Note also that if the type of the advertisement is Summary link or AS
external link ...
... Sending Link State Acknowledgment packets ...
...
Each newly received link state advertisement must be acknowledged. This
is usually done by sending Link State Acknowledgment packets. However,
...
... Each newly received link state advertisement must be acknowledged. This
is usually done by sending Link State Acknowledgment packets. However,
acknowledgments can also be accomplished implicitly by sending Link
State Update ...
... is usually done by sending Link State Acknowledgment packets. However,
acknowledgments can also be accomplished implicitly by sending Link
State Update packets (see step 7a of Section 13).
...
... Update packets (see step 7a of Section 13).
Many acknowledgments may be grouped together into a single Link State
Acknowledgment packet. Such a packet is sent back out the interface
...
... Direct acknowledgments are sent to a particular neighbor in response to
the receipt of duplicate link state advertisements. These
acknowledgments are sent as unicasts, and are sent immediately when the
...
... duplicate is received.
The precise procedure for sending Link State Acknowledgment packets is
described in Table 19. The circumstances surrounding the receipt of the
advertisement are listed in the left column. The acknowledgment action
...
... current instance of
the advertisement in
the link state
database (see
Section 13, step 4).
...
... Section 13, step 4).
Table 19: Sending link state acknowledgements.
Delayed acknowledgments must be delivered to all adjacent routers ...
... broadcast networks, this is
accomplished by sending the delayed Link State Acknowledgment packets as
multicasts. The Destination IP address used depends on the ...
... is slightly
different is because they perform differently during the flooding of
link state advertisements (see Section 13.3, step 4).
...
... Retransmitting link state advertisements ...
...
Advertisements flooded out an adjacency are placed on the adjacency's
Link state retransmission list. In order to ensure that flooding is
...
... may be affected.
Several retransmitted advertisements may fit into a single Link State
Update packet. When advertisements are to be retransmitted, only the
number fitting in a single ...
...
Update packet. When advertisements are to be retransmitted, only the
number fitting in a single Link State Update packet should be
transmitted. Another packet of retransmissions ...
... OSPF's Hello Protocol. When the adjacency is
destroyed, the Link state retransmission list is cleared.
...
... Receiving link state acknowledgments ...
...
Many consistency checks have been made on a received Link State
Acknowledgment packet before it is handed to the flooding procedure. In
...
... are performed:
o Does the advertisement acknowledged have an instance on the Link
state retransmission list for the neighbor? If not, examine the
...
... Aging The Link State Database ...
...
Each link state advertisement has an age field. The age is expressed in
seconds. An advertisement's age field is incremented while it is
contained in a router ...
... contained in a router's database. Also, when copied into a Link State
Update Packet for flooding out a particular ...
... routing table
calculation. As a router ages its link state database, an
advertisement's age may reach MaxAge.[17] At this time, the router ...
... Database summary list for a newly adjacent neighbor is formed,
any MaxAge advertisements present in the link state database are added
to the neighbor's Link state ...
... link state database are added
to the neighbor's Link state retransmission list instead of the
neighbor's ...
... A MaxAge advertisement is removed entirely from the router's link state
database when a) it is no longer contained on any neighbor Link state
...
... router's link state
database when a) it is no longer contained on any neighbor Link state
retransmission lists and b) none of the router's ...
... Exchange or Loading.
When, in the process of aging the link state database, an
advertisement's age hits a multiple of CheckAge, its checksum should be
...
... the MaxAge advertisement is removed from the router's link state
database as soon as a) it is no longer contained on any neighbor Link
state retransmission ...
... router's link state
database as soon as a) it is no longer contained on any neighbor Link
state retransmission lists and b) none of the router's neighbors ...
...
A router may only prematurely age its own (self-originated) link state
advertisements. These are the link state advertisements having the
router's own ...
... A router may only prematurely age its own (self-originated) link state
advertisements. These are the link state advertisements having the
router's own OSPF Router ID in the Advertising ...
... Virtual Links ...
... Autonomous System will become unreachable. To
establish/maintain connectivity of the backbone, virtual links can be
configured through non-backbone areas. Virtual links ...
... virtual links can be
configured through non-backbone areas. Virtual links serve to connect
separate components of the backbone. The two endpoints ...
... separate components of the backbone. The two endpoints of a virtual
link are area border routers. The virtual link must be configured in
...
... of a virtual
link are area border routers. The virtual link must be configured in
both routers. The configuration information ...
... backbone area the two routers have in common (called the transit area).
Virtual links cannot be configured through stub areas (see Section 3.6).
The virtual link ...
... Virtual links cannot be configured through stub areas (see Section 3.6).
The virtual link is treated as if it were an unnumbered point-to-point
network (belonging to the ...
... backbone) joining the two area border routers.
An attempt is made to establish an adjacency over the virtual link.
When this adjacency is established, the virtual link will be included in
...
... An attempt is made to establish an adjacency over the virtual link.
When this adjacency is established, the virtual link will be included in
backbone router ...
... will be included in
backbone router links advertisements, and OSPF packets pertaining to the
backbone area will ...
... In each endpoint router, the cost and viability of the virtual link is
discovered by examining the routing table entry for the other endpoint ...
... area). Actually, there may be a separate routing table entry for each
Type of Service. These are called the virtual link's corresponding
routing table entries. The Interface Up event occurs for a virtual ...
... 's corresponding
routing table entries. The Interface Up event occurs for a virtual link
when its corresponding TOS 0 routing table ...
... routing table
entry becomes unreachable.[18] In other words, the virtual link's
viability is determined by the existence of an intra-area path, through
...
... the transit area, between the two endpoints. The other details
concerning virtual links are as follows:
o AS ...
...
o AS external links are NEVER flooded over virtual adjacencies. This
would be duplication of effort, since the same AS external links ...
... links are NEVER flooded over virtual adjacencies. This
would be duplication of effort, since the same AS external links are
already flooded throughout the virtual link's transit area. For
...
... AS external links are
already flooded throughout the virtual link's transit area. For
this same reason, AS external link ...
... link's transit area. For
this same reason, AS external link advertisements are not summarized
over virtual adjacencies during the database exchange process.
...
... database exchange process.
o The cost of a virtual link is NOT configured. It is defined to be
the cost of the intra-area path between the two defining area border
routers ...
... the cost of the intra-area path between the two defining area border
routers. This cost appears in the virtual link's corresponding
routing table entry. When the cost of a virtual link ...
... link's corresponding
routing table entry. When the cost of a virtual link changes, a new
router links ...
... area.
o Just as the virtual link's cost and viability are determined by the
routing table build process (through construction of the routing
table ...
... neighbor's IP address.
These are used when sending protocol packets over the virtual link.
o In each endpoint ...
... links advertisement for the backbone, the
virtual link is represented as a link having link type 4, Link ID ...
... virtual link is represented as a link having link type 4, Link ID
set to the virtual neighbor ...
... link is represented as a link having link type 4, Link ID
set to the virtual neighbor's OSPF Router ...
... set to the virtual neighbor's OSPF Router ID and Link Data set to
the virtual interface's IP address ...
... links advertisement.
o When virtual links are configured for the backbone, information
concerning backbone networks ...
... backbone networks should not be condensed before being
summarized for the transit areas. In other words, each backbone
network should be advertised in a separate summary link
advertisement, regardless of the backbone's configured area address ...
... ranges. See Section 12.4.3 for more information.
o The time between link state retransmissions, RxmtInterval, is
configured for a virtual link ...
... link state retransmissions, RxmtInterval, is
configured for a virtual link. This should be well over the
expected round-trip delay between the two routers ...
... round-trip delay between the two routers. This may be hard
to estimate for a virtual link. It is better to err on the side of
making it too large.
...
... OSPF routing table calculation. Using its
attached areas' link state databases as input, a router runs the
...
... routing table step by step. At each
step, the router must access individual pieces of the link state
databases (e.g., a router ...
...
databases (e.g., a router links advertisement originated by a certain
router). This access is performed by the lookup function discussed in
...
... lookup function discussed in
Section 12.2. The lookup process may return a link state advertisement
whose LS age is equal to MaxAge. Such an advertisement should not be
used in the routing table ...
... calculated in this step. This step is described in two parts. At
first the tree is constructed by only considering those links
between routers and transit networks ...
... (3) The inter-area routes are calculated, through examination of summary
link advertisements. If the router is attached to multiple areas
(i.e., it is an area border router ...
... (i.e., it is an area border router), only backbone summary link
advertisements are examined.
...
... (4) For those routing entries whose next hop is over a virtual link, a
real (physical) next hop ...
... networks. This step only
concerns routers having configured virtual links.
(5) Routes to external destinations ...
... destinations are calculated, through examination
of AS external link advertisements. The location of the AS boundary
routers ...
... routers (which originate the AS external link advertisements) has
been determined in steps 2-4.
...
... route will cause an area border router to
originate new summary link advertisements (see Section 12.4). See
Section 16.7 for a complete list of the OSPF protocol actions resulting
...
... root.[20] The formation of the shortest path
tree is done here in two stages. In the first stage, only links between
routers and transit networks are considered. Using the Dijkstra
...
... are considered. Using the Dijkstra
algorithm, a tree is formed from this subset of the link state database.
In the second stage, leaves are added to the tree by considering the
...
... .
In the second stage, leaves are added to the tree by considering the
links to stub networks.
...
...
The procedure will be explained using the graph terminology that was
introduced in Section 2. The area's link state database is represented
as a directed graph. The graph's vertices are routers, transit networks ...
... transit vertices (routers and transit networks) and their connecting
links. Throughout the shortest path calculation, the following data is
also associated with each transit vertex:
...
... Designated Router.
A link state advertisement
Each transit vertex has an associated link state advertisement. For
...
... A link state advertisement
Each transit vertex has an associated link state advertisement. For
router vertices, this is a router ...
... router vertices, this is a router links advertisement. For transit
networks, this is a network ...
... networks, this is a network links advertisement (which is actually
originated by the network's Designated Router ...
... network's Designated Router). In any case, the
advertisement's Link State ID is always equal to the above Vertex
ID.
...
... Distance from root
The link state cost of the current shortest path(s) from the root to
the vertex. The link state ...
... link state cost of the current shortest path(s) from the root to
the vertex. The link state cost of a path is calculated as the sum
of the costs of the path's constituent links (as advertised in
...
... the vertex. The link state cost of a path is calculated as the sum
of the costs of the path's constituent links (as advertised in
router links ...
... router links and network links advertisements). One path is said to
be "shorter" than another if it has a smaller link state cost.
...
... network links advertisements). One path is said to
be "shorter" than another if it has a smaller link state cost.
The first stage of the procedure can now be summarized as follows. At
...
... The following steps describe the first stage in detail. Remember that
we are computing the shortest path tree for Area A. All references to
link state database lookup below are from Area A's database.
...
...
(2) Call the vertex just added to the tree vertex V. Examine the link
state advertisement associated with vertex V. This is a lookup in
the area link state database ...
... link
state advertisement associated with vertex V. This is a lookup in
the area link state database based on the Vertex ID. Each link
described by the advertisement gives the cost to an adjacent vertex.
...
... lookup in
the area link state database based on the Vertex ID. Each link
described by the advertisement gives the cost to an adjacent vertex.
For each described link ...
... link
described by the advertisement gives the cost to an adjacent vertex.
For each described link, (say it joins vertex V to vertex W):
(a) If this is a link ...
... link, (say it joins vertex V to vertex W):
(a) If this is a link to a stub network, examine the next link in
...
... (a) If this is a link to a stub network, examine the next link in
V's advertisement. Links to stub networks ...
... network, examine the next link in
V's advertisement. Links to stub networks will be considered in
the second stage of the shortest path calculation.
...
... router or transit network).
Look up the vertex W's link state advertisement (router links or
...
... links or
network links) in Area A's link state database. If the
advertisement does not exist, or its age is equal to MaxAge, or
...
... network links) in Area A's link state database. If the
advertisement does not exist, or its age is equal to MaxAge, or
it does not have a link ...
... link state database. If the
advertisement does not exist, or its age is equal to MaxAge, or
it does not have a link back to vertex V, examine the next link
in V's advertisement. Both ends of a link ...
... advertisement does not exist, or its age is equal to MaxAge, or
it does not have a link back to vertex V, examine the next link
in V's advertisement. Both ends of a link must advertise the
...
... link back to vertex V, examine the next link
in V's advertisement. Both ends of a link must advertise the
link before it will be used for data traffic ...
... in V's advertisement. Both ends of a link must advertise the
link before it will be used for data traffic.[21]
...
... (c) If vertex W is already on the shortest-path tree, examine the
next link in the advertisement.
(d) If the cost of the link ...
... link in the advertisement.
(d) If the cost of the link (from V to W) is LSInfinity, the link
should not be used for data traffic ...
...
(d) If the cost of the link (from V to W) is LSInfinity, the link
should not be used for data traffic. In this case, examine the
...
... should not be used for data traffic. In this case, examine the
next link in the advertisement.
(e) Calculate the link state ...
... link in the advertisement.
(e) Calculate the link state cost D of the resulting path from the
root to vertex W. D is equal to the sum of the link state ...
... link state cost D of the resulting path from the
root to vertex W. D is equal to the sum of the link state cost
of the (already calculated) shortest path to vertex V and the
advertised cost of the link ...
... link state cost
of the (already calculated) shortest path to vertex V and the
advertised cost of the link between vertices V and W. If D is:
o Greater than the value that already appears for vertex W on
...
... o Greater than the value that already appears for vertex W on
the candidate list, then examine the next link.
o Equal to the value that appears for vertex W on the the
...
... candidate list, calculate the set of next hops that result
from using the advertised link. Input to this calculation
is the destination (W), and its parent (V). This
...
... list of next hops that result from using the advertised
link, setting the next hop values for W accordingly. The
next hop ...
... Options field found in the associated router links advertisement is
copied into the routing table entry's Optional capabilities field.
...
... routing table entry is
added/modified, the Options found in the associated router links
advertisement is copied into the routing table entry's Optional
...
... IP network number, which can be obtained by masking the Vertex ID
(Link State ID) with its associated subnet mask (found in the
associated network ...
... subnet mask (found in the
associated network links advertisement). If the routing table entry
already exists (i.e., there is already an intra-area ...
... newly found path is just as short and the current routing table
entry's Link State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement ...
... routing table
entry's Link State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement.
...
... Link State Origin has a smaller Link State ID than the newly
added vertex' link state advertisement.
If there is no routing table ...
... IP network should be added. The
routing table entry's Link State origin should be set to the newly
added vertex' link state advertisement.
...
... routing table entry's Link State origin should be set to the newly
added vertex' link state advertisement.
(5) Iterate the algorithm ...
... discarded. For each reachable router vertex (call it V), the associated
router links advertisement is found in the link state database. Each
stub network ...
... vertex (call it V), the associated
router links advertisement is found in the link state database. Each
stub network link ...
... link state database. Each
stub network link appearing in the advertisement is then examined, and
the following steps are executed:
...
...
(1) If the cost of the stub network link is LSInfinity, the link should
not be used for data traffic ...
... (1) If the cost of the stub network link is LSInfinity, the link should
not be used for data traffic. In this case, go on to examine the
...
... data traffic. In this case, go on to examine the
next stub network link in the advertisement.
(2) Otherwise, Calculate the distance D of stub network ...
... router vertex
(calculated in stage 1), plus the stub network link's advertised
cost. Compare this distance to the current best cost to the stub
network ...
... routing
table entry. If the calculated distance D is larger, go on to
examine the next stub network link in the advertisement.
(3) If this step is reached, the stub network ...
... next hops that would result from
using the stub network link. This calculation is shown in Section
16.1.1; input to this calculation is the destination (the stub
...
... next hops. In
this case, the routing table already has a Link State origin. If
this Link State origin is a router ...
... routing table already has a Link State origin. If
this Link State origin is a router links advertisement whose Link
State ...
... this Link State origin is a router links advertisement whose Link
State ID is smaller than V's Router ID, reset the Link State ...
... Link State origin is a router links advertisement whose Link
State ID is smaller than V's Router ID, reset the Link State origin
...
... links advertisement whose Link
State ID is smaller than V's Router ID, reset the Link State origin
to V's router links ...
... Link State origin
to V's router links advertisement.
Otherwise D is smaller than the routing table ...
... next hops to the newly
calculated set. Set the routing table entry's Link State origin to
V's router links ...
... Link State origin to
V's router links advertisement. Then go on to examine the next stub
network link ...
... links advertisement. Then go on to examine the next stub
network link.
For all routing table ...
... associated area will be set to Area A and the path type will be set to
intra-area. When the list of reachable router links is exhausted, the
second stage is completed. At this time, all intra-area routes
...
... is used, an identical tree must be produced. For this reason, it is
important to note that links between transit vertices must be
bidirectional in ordered to be included in the above tree. It should
...
... is then determined by examining the
destination's router links advertisement. For each link in the
advertisement that points back to the parent network ...
... destination's router links advertisement. For each link in the
advertisement that points back to the parent network, the link ...
... link in the
advertisement that points back to the parent network, the link's Link
Data field provides the ...
... advertisement that points back to the parent network, the link's Link
Data field provides the IP address of a ...
...
The inter-area routes are calculated by examining summary link
advertisements. If the router has active ...
... active attachments to multiple areas,
only backbone summary link advertisements are examined. Routers
attached to a single area examine that area's summary links ...
... link advertisements are examined. Routers
attached to a single area examine that area's summary links. In either
case, the summary links examined below are all part of a single area's
...
... attached to a single area examine that area's summary links. In either
case, the summary links examined below are all part of a single area's
link state database (call it Area A).
...
... . In either
case, the summary links examined below are all part of a single area's
link state database (call it Area A).
Summary link ...
... link state database (call it Area A).
Summary link advertisements are originated by the area border routers.
Each summary link ...
... link advertisements are originated by the area border routers.
Each summary link advertisement in Area A is considered in turn.
Remember that the destination described by a summary link ...
... link advertisement in Area A is considered in turn.
Remember that the destination described by a summary link advertisement
is either a network (type 3 summary link ...
... link advertisement
is either a network (type 3 summary link advertisements) or an AS
boundary router (type 4 summary link ...
... link advertisements) or an AS
boundary router (type 4 summary link advertisements). For each summary
link advertisement:
...
... router (type 4 summary link advertisements). For each summary
link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, then
...
...
(3) If the collection of destinations described by the summary link
falls into one of the router's configured area address ...
... address range is active, the
summary link should be ignored. Active means that there are one or
more reachable (by intra-area ...
...
This step is only necessary in area-border routers having configured
virtual links. In these routers, some of the routing table entries may
...
... next hops. That is, one or more of the next hops installed
in Sections 16.1 and 16.2 may be over a virtual link. However, when
forwarding data traffic to a destination ...
... routing table
entry's next hops is a virtual link. This is determined by the
following combination: the routing table entry's path type is either
...
... backbone, yet the next hop belongs to a different area
(the virtual link's transit area).
Let N be the above entry's destination ...
...
Let N be the above entry's destination, and A the virtual link's transit
area. The real next hop (and new distance) is calculated as follows.
...
... counter, and set the real next hop NH to null.
Then, look up all the summary link advertisements for N in area A's
database, performing the following steps for each advertisement:[23]
...
... advertised by BR (call it Y) to the destination is LSInfinity,
examine the next summary link advertisement. Else, the cost to
destination N through area border router ...
... next hops are resolved, traffic that was originally
scheduled to go over the virtual link may instead take a different path
through the virtual link's transit area. In other words, virtual links ...
... scheduled to go over the virtual link may instead take a different path
through the virtual link's transit area. In other words, virtual links
allow transit traffic ...
... link may instead take a different path
through the virtual link's transit area. In other words, virtual links
allow transit traffic to be forwarded through an area, but do not
...
... backbone into two separate pieces. To maintain connectivity of the
backbone, a virtual link has been configured between routers RT1 and
RT4. On the right side of the figure, network ...
... router
RT4 and router RT5 will inject summary link advertisements for network
N1 into Area 1.
...
... has been calculated for the backbone,
router RT1 (one end of the virtual link) will have selected router RT4
as the virtual next hop ...
... through the transit Area 1, but the actual path the data traffic takes
does not follow the virtual link.
...
...
AS external routes are calculated by examining AS external link
advertisements. Each of the AS external link ...
... link
advertisements. Each of the AS external link advertisements is
considered in turn. Most AS external advertisements describe routes to
...
... destination =
DefaultDestination). For each AS external link advertisement:
(1) If the cost specified by the advertisement is LSInfinity, then
...
... the ASBR itself), and the cost specified in the advertisement Y. X
is in terms of the link state metric, and Y is a Type 1 or 2
external metric.
...
... then the path-type is set to type 1 external and the cost is equal
to X+Y. If the external metric type is 2, the the path-type is set
to type 2 external, the link state component of the route's cost is
X, and the Type 2 cost is Y.
...
... Incremental updates --- summary links ...
...
When a new summary link advertisement is received, it is not necessary
to recalculate the entire routing table. Call the destination ...
... routing table. Call the destination described
by the summary link advertisement N, and let A be the area to which the
advertisement belongs.
...
... routing table entry for N. If the next hop to N is a
virtual link through Area A (this means that the entry's associated area
is the backbone, and the listed next hop ...
... backbone, the
procedure in Section 16.2 will have to be performed, but only for those
summary link advertisements whose destination is N. Before this
procedure is performed, the present routing table ...
...
When a new AS external link advertisement is received, it is not
necessary to recalculate the entire routing table. Call the destination ...
... destination
described by the AS external link advertisement N. If there is already
an intra-area or inter-area ...
... Otherwise, the procedure in Section 16.4 will have to be performed, but
only for those AS external link advertisements whose destination is N.
Before this procedure is performed, the present routing table ...
... router, and this is not simply a change of AS external routes, new
summary link advertisements may have to be generated (potentially
one for each attached area, including the backbone). See Section
...
...
o A routing table entry associated with a configured virtual link has
changed. The destination of such a routing table ...
... routing table entry is an area
border router. The change indicates a modification to the virtual
link's cost or viability.
If the entry indicates that the area border router ...
... area border router is newly
reachable (via TOS 0), the corresponding virtual link is now
operational. An Interface Up event should be generated for the
...
... operational. An Interface Up event should be generated for the
virtual link, which will cause a virtual adjacency to begin to form
(see Section 10.3). At this time the virtual interface's IP address ...
... area border router is no longer
reachable (via TOS 0), the virtual link and its associated adjacency
should be destroyed. This means an Interface Down event should be
...
... should be destroyed. This means an Interface Down event should be
generated for the associated virtual link.
If the cost of the entry has changed, and there is a fully
...
... If the cost of the entry has changed, and there is a fully
established virtual adjacency, a new router links advertisement for
the backbone must be originated. This in turn may cause further
...
... in the Options field of its
router links advertisement.
The above sections detailing the routing table ...
... TOS X, only TOS X
metrics can be used. Any link state advertisement may specify a
separate cost for each TOS (a cost for TOS ...
... zero TOS is not handled) by clearing the T-bit in the link state
advertisement's Option field. Such advertisements are not used when
calculating routes for non-zero TOS ...
... those having the T-bit reset in their router links advertisements.
Such routers should never be added to the Dijktra algorithm ...
... candidate list, nor should their router links advertisements be
examined when adding the stub networks to the tree ...
... Inter-area paths are the concatenation of a path to an area border
router with a summary link. When calculating TOS X routes, both
path components must also specify TOS ...
... route to the destination. Note
that this means that summary link advertisements having the T-bit
reset in their Options field ...
... concatenation of a path to an
area border router with a summary link. As with inter-area routes,
only TOS ...
... destination. Note that this
means that AS external link advertisements having the T-bit reset in
their Options field ...
... It is possible for all of a router's interfaces to be unnumbered point-to-point links. In this case, an IP address must be assigned to the router. This address ...
... networks, the lower level protocols indicate whether the neighbor is up and running. Likewise, existence of the neighbor on virtual links is indicated by the routing table calculation. However, in both these cases, the Hello Protocol is still used. This ensures that communication between the neighbors ...
... MaxAgeDiff is an architectural constant. It indicates the maximum dispersion of ages, in seconds, that can occur for a single
link state instance as it is flooded throughout the routing domain. If two advertisements differ by more than this, they are assumed to be different instances of the same advertisement. This
can occur when a router ...
... restarts, and loses track of its previous sequence number. In this case, since the two advertisements have the same sequence number, it is not possible to determine which link state is actually newer. If the wrong advertisement is accepted as newer, the originating router will originate another instance. See Section 13.4 for further details. ...
... lookup must be done based on partial information. This is during the routing table calculation, when a network links advertisement must be found based solely on its Link State ID. The lookup in this case is still well defined, since no two network ...
... routing table calculation, when a network links advertisement must be found based solely on its Link State ID. The lookup in this case is still well defined, since no two network advertisements can have the same Link ...
... Link State ID. The lookup in this case is still well defined, since no two network advertisements can have the same Link State ID. ...
... This is how the Link state request list is emptied, which eventually causes the neighbor state to transition to Full. See Section 10.9 for more details. ...
... data traffic will flow between a pair of neighboring routers, their link state databases must be synchronized. Before synchronization (neighbor ...
... As a result of this clause, when a virtual link exists between the calculating router and an AS boundary router ...
... AS boundary router, the intra-area path through the virtual link's transit area is always preferred over the virtual link itself. ...
... router, the intra-area path through the virtual link's transit area is always preferred over the virtual link itself. ...
...
This appendix describes the format of OSPF protocol packets and OSPF
link state advertisements. The OSPF protocol runs directly over the IP
network ...
... domain. It is contained both in OSPF protocol packets and in
OSPF link state advertisements.
...
... advertisements.
OSPF packet formats are detailed in Section A.3. A description of OSPF
link state advertisements appears in Section A.4.
...
... MTU. The OSPF packet types that are likely to be large (Database
Description Packets, Link State Request, Link State Update, and Link
State ...
... OSPF packet types that are likely to be large (Database
Description Packets, Link State Request, Link State Update, and Link
State Acknowledgment packets) can usually be split into several separate
...
... Link State Request, Link State Update, and Link
State Acknowledgment packets) can usually be split into several separate
protocol packets, without loss of functionality. This is recommended;
IP ...
... fragmentation should be avoided whenever possible. Using this
reasoning, an attempt should be made to limit the sizes of packets sent
over virtual links to 576 bytes. However, if necessary, the length of
OSPF packets can be up to 65,535 bytes (including the IP header).
...
... options field is present in OSPF Hello packets, Database
Description packets and all link state advertisements. The options
field enables OSPF routers to support (or not support) optional
...
... Two capabilities are currently defined. For each capability, the effect
of the capability's appearance (or lack of appearance) in Hello packets,
Database Description packets and link state advertisements is specified
below. For example, the external routing capability (below called the
...
... (i.e. clear) the unassigned part of the capability field when sending
Hello packets or Database Description packets and when originating link
state advertisements.
Additional capabilities may be assigned in the future. Routers ...
...
encountering unrecognized capabilities in received Hello Packets,
Database Description packets or link state advertisements should ignore
the capability and process the packet/advertisement normally.
...
... bit in a router links advertisement causes the router to be skipped
when building a non-zero ...
... link advertisement or an
AS external link advertisement indicates that the advertisement is
describing a TOS 0 route ...
... bit
AS external link advertisements are not flooded into/through OSPF
stub areas (see Section 3.6). The E-bit ...
... router will neither send nor receive AS external link state
advertisements on that interface (in other words, the interface
...
... OSPF packet types (other than the OSPF Hello packets) deal with
lists of link state advertisements. For example, Link State Update
...
... OSPF Hello packets) deal with
lists of link state advertisements. For example, Link State Update
packets implement the flooding ...
... domain. Because of this, OSPF protocol packets cannot be parsed
unless the format of link state advertisements is also understood. The
format of Link state advertisements is described in Section A.4.
...
... unless the format of link state advertisements is also understood. The
format of Link state advertisements is described in Section A.4.
The receive processing of OSPF packets ...
... All OSPF packets are associated with a single area. Most travel a
single hop only. Packets travelling over a virtual link are
labelled with the backbone area ID ...
... OSPF packet type 1. These packets are sent
periodically on all interfaces (including virtual links) in order to
establish and maintain neighbor relationships. In addition, Hellos are
...
... The format of the Database Description packet is very similar to both
the Link State Request and Link State Acknowledgment packets. The main
part of all three is a list of items, each item describing a piece of
...
... Database Description packet is very similar to both
the Link State Request and Link State Acknowledgment packets. The main
part of all three is a list of items, each item describing a piece of
the topological database ...
... +- -+
| A |
+- Link State Advertisement -+
| Header |
...
... The rest of the packet consists of a (possibly partial) list of the
topological database's pieces. Each link state advertisement in the
database is described by its link state ...
... 's pieces. Each link state advertisement in the
database is described by its link state header. The link state header ...
... database is described by its link state header. The link state header
is documented in Section A.4.1. It contains all the information
...
... A.3.4 The Link State Request packet ...
...
Link State Request packets are OSPF packet type 3. After exchanging
Database Description packets with a neighboring ...
... router may
find that parts of its topological database are out of date. The Link
State Request packet is used to request the pieces of the neighbor's
database that are more up to date. Multiple ...
... Request packet is used to request the pieces of the neighbor's
database that are more up to date. Multiple Link State Request packets
may need to be used. The sending of Link State Request packets is the
...
... database that are more up to date. Multiple Link State Request packets
may need to be used. The sending of Link State Request packets is the
last step in bringing up an adjacency.
...
...
A router that sends a Link State Request packet has in mind the precise
instance of the database pieces it is requesting (defined by LS sequence
number ...
... instances in response.
The sending of Link State Request packets is documented in Section 10.9.
The reception of Link State Request packets is documented in Section
...
... The sending of Link State Request packets is documented in Section 10.9.
The reception of Link State Request packets is documented in Section
10.7.
...
... | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router ...
... | ... |
Each advertisement requested is specified by its LS type, Link State ID,
and Advertising Router. This uniquely identifies the advertisement, but
...
... and Advertising Router. This uniquely identifies the advertisement, but
not its instance. Link State Request packets are understood to be
requests for the most recent instance (whatever that might be).
...
... A.3.5 The Link State Update packet ...
... OSPF packet type 4. These packets
implement the flooding of link state advertisements. Each Link State
Update packet carries a collection of ...
... implement the flooding of link state advertisements. Each Link State
Update packet carries a collection of link state advertisements one hop
...
... . Each Link State
Update packet carries a collection of link state advertisements one hop
further from its origin. Several link state advertisements may be
...
... Update packet carries a collection of link state advertisements one hop
further from its origin. Several link state advertisements may be
included in a single packet.
...
... may be
included in a single packet.
Link State Update packets are multicast on those physical ...
... broadcast. In order to make the flooding procedure
reliable, flooded advertisements are acknowledged in Link State
Acknowledgment packets. If retransmission of certain advertisements is
...
... of certain advertisements is
necessary, the retransmitted advertisements are always carried by
unicast Link State Update packets. For more information on the reliable
flooding of ...
... Update packets. For more information on the reliable
flooding of link state advertisements, consult Section 13.
0 1 2 3
...
... | |
+- +-+
| Link state advertisements |
+- +-+
| ... |
...
...
# advertisements
The number of link state advertisements included in this update.
...
... update.
The body of the Link State Update packet consists of a list of link
state advertisements. Each advertisement begins with a common 20 byte
...
... The body of the Link State Update packet consists of a list of link
state advertisements. Each advertisement begins with a common 20 byte
header, the link state advertisement ...
... packet consists of a list of link
state advertisements. Each advertisement begins with a common 20 byte
header, the link state advertisement header. This header is described
...
... . This header is described
in Section A.4.1. Otherwise, the format of each of the five types of
link state advertisements is different. Their formats are described in
Section A.4.
...
... A.3.6 The Link State Acknowledgment packet ...
... Acknowledgment Packets are OSPF packet type 5. To make the
flooding of link state advertisements reliable, flooded advertisements
are explicitly acknowledged. This acknowledgment is accomplished
through the sending and receiving ...
... are explicitly acknowledged. This acknowledgment is accomplished
through the sending and receiving of Link State Acknowledgment packets.
Multiple link state advertisements can be acknowledged in a single
...
... receiving of Link State Acknowledgment packets.
Multiple link state advertisements can be acknowledged in a single
packet.
...
... state of the sending interface and the source of the
advertisements being acknowledged, a Link State Acknowledgment packet is
sent either to the multicast address AllSPFRouters, to the multicast
address ...
... multicast address AllSPFRouters, to the multicast
address AllDRouters, or as a unicast. The sending of Link State
Acknowledgement packets is documented in Section 13.5. The reception of
Link State Acknowledgement packets is documented in Section 13.7.
...
... . The sending of Link State
Acknowledgement packets is documented in Section 13.5. The reception of
Link State Acknowledgement packets is documented in Section 13.7.
The format of this packet is similar to that of the Data Description
...
...
The format of this packet is similar to that of the Data Description
packet. The body of both packets is simply a list of link state
advertisement headers.
...
... +- -+
| A |
+- Link State Advertisement -+
| Header |
...
...
Each acknowledged link state advertisement is described by its link
state header. The link state ...
...
Each acknowledged link state advertisement is described by its link
state header. The link state header ...
... link state advertisement is described by its link
state header. The link state header is documented in Section A.4.1. It
contains all the information required to uniquely identify both the
...
... A.4 Link state advertisement formats ...
...
There are five distinct types of link state advertisements. Each link
state advertisement begins with a standard 20-byte link state ...
...
There are five distinct types of link state advertisements. Each link
state advertisement begins with a standard 20-byte link state header ...
... link state advertisements. Each link
state advertisement begins with a standard 20-byte link state header.
This header ...
... This header is explained in Section A.4.1. Succeeding sections then
diagram the separate link state advertisement types.
Each link state advertisement ...
... link state advertisement types.
Each link state advertisement describes a piece of the OSPF routing
...
... domain. Every router originates a router links advertisement. In
addition, whenever the router is elected Designated Router ...
... Designated Router, it
originates a network links advertisement. Other types of link state
advertisements may also be originated (see Section 12.4). All link
state advertisements are then flooded throughout the OSPF ...
... originates a network links advertisement. Other types of link state
advertisements may also be originated (see Section 12.4). All link
state advertisements are then flooded throughout the OSPF routing ...
... network links advertisement. Other types of link state
advertisements may also be originated (see Section 12.4). All link
state advertisements are then flooded throughout the OSPF routing
...
... algorithm is reliable, ensuring that all routers
have the same collection of link state advertisements. (See Section 13
for more information concerning the flooding algorithm ...
... flooding algorithm). This
collection of advertisements is called the link state (or topological)
database.
...
... database.
From the link state database, each router constructs a shortest path
tree with itself as root ...
... A.4.1 The Link State Advertisement header ...
...
All link state advertisements begin with a common 20 byte header. This
header contains enough information to uniquely identify the
...
... . This
header contains enough information to uniquely identify the
advertisement (LS type, Link State ID, and Advertising Router).
Multiple instances of the link state advertisement ...
... Link State ID, and Advertising Router).
Multiple instances of the link state advertisement may exist in the
routing domain at the same time. It is then necessary to determine
...
... sequence number and LS checksum fields that are also contained
in the link state advertisement header.
...
... | LS age | Options | LS type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router ...
...
LS age
The time in seconds since the link state advertisement was
originated.
...
...
LS type
The type of the link state advertisement. Each link state type has
a separate advertisement format. The link state ...
... LS type
The type of the link state advertisement. Each link state type has
a separate advertisement format. The link state types are as
...
... link state advertisement. Each link state type has
a separate advertisement format. The link state types are as
follows (see Section 12.1.3 for further explanation):
...
... external link
Link State ID
This field identifies the portion of the internet environment that
...
... depend on the advertisement's LS type. For example, in network
links advertisements the Link State ID is set to the IP interface
...
... network's
IP address can be derived). The Link State ID is further discussed
in Section 12.1.4.
...
... The Router ID of the router that originated the link state
advertisement. For example, in network links advertisements this
...
... router that originated the link state
advertisement. For example, in network links advertisements this
field is set to the Router ID of the network ...
... LS sequence number
Detects old or duplicate link state advertisements. Successive
instances of a link state advertisement are given successive LS
...
... Detects old or duplicate link state advertisements. Successive
instances of a link state advertisement are given successive LS
sequence numbers. See Section 12.1.6 for more details.
...
... checksum
The Fletcher checksum of the complete contents of the link state
advertisement. See Section 12.1.7 for more details.
length
...
...
length
The length in bytes of the link state advertisement. This includes
the 20 byte link state header ...
... The length in bytes of the link state advertisement. This includes
the 20 byte link state header.
...
...
Router links advertisements are the Type 1 link state advertisements.
Each router in an area originates a router ...
... Each router in an area originates a router links advertisement. The
advertisement describes the state and cost of the router ...
... advertisement describes the state and cost of the router's links (or
interfaces) to the area. All of the router's ...
... (or
interfaces) to the area. All of the router's links to the area must be
described in a single router links ...
... links to the area must be
described in a single router links advertisement. For details
concerning the construction of router links ...
... links advertisement. For details
concerning the construction of router links advertisements, see Section
12.4.1.
...
... In router links advertisements, the Link State ID field is set to the
router's OSPF Router ID. The T- ...
... | LS age | Options | 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link State ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Advertising Router ...
... checksum | length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0 |E|B| 0 | # links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID ...
... links |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ...
... Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | # TOS ...
... TOS | 0 | metric |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link ...
... Link ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Link Data |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
...
... links
The number of router links described by this advertisement. This
must be the total collection of router links ...
... links described by this advertisement. This
must be the total collection of router links to the area.
The following fields are used to describe each router ...
...
The following fields are used to describe each router link. Each router
link is typed (see the below ...
... Type field). The type field indicates the
kind of link being described. It may be a link to a transit network, to
...
... type field indicates the
kind of link being described. It may be a link to a transit network, to
another router ...
... network. The values of all the other fields
describing a router link depend on the link's type. For example, each
link has an associated ...
... describing a router link depend on the link's type. For example, each
link has an associated 32-bit ...
... link depend on the link's type. For example, each
link has an associated 32-bit data field. For links ...
... link has an associated 32-bit data field. For links to stub networks
this field specifies the network ...
... this field specifies the network's IP address mask. For the other link
types the Link Data specifies the router's associated IP interface ...
... network's IP address mask. For the other link
types the Link Data specifies the router's associated IP interface
...
... Type
A quick description of the router link. One of the following. Note
that host routes are classified as links ...
... link. One of the following. Note
that host routes are classified as links to stub networks whose
network ...
... Link ID
Identifies the object that this router link connects to. Value
depends on the link's type. When connecting to an object that also
...
... router link connects to. Value
depends on the link's type. When connecting to an object that also
originates a link state advertisement (i.e., another router ...
... depends on the link's type. When connecting to an object that also
originates a link state advertisement (i.e., another router or a
transit network ...
... Link ID is equal to the other advertisement's
Link State ID. This provides the key for looking up said
advertisement in the link state database. See Section 12.2 for more
...
... Link State ID. This provides the key for looking up said
advertisement in the link state database. See Section 12.2 for more
details.
...
... stub network, it specifies the network mask. For the other link
types it specifies the router's associated IP interface address ...
... #metrics
The number of different TOS metrics given for this link, not
counting the required metric for TOS 0. For example, if no
...
... TOS 0.
For each link, separate metrics may be specified for each Type of
Service (TOS). The metric for TOS ...