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... Internet Engineering Task Force. It has been designed expressly for the
internet environment, including explicit support for IP subnetting,
TOS-based routing and the tagging of externally-derived ...
... authentication of routing
updates, and utilizes IP multicast when sending/receiving the updates.
In addition, much work has been done to produce a protocol that responds
...
...
OSPF routes IP packets based solely on the destination IP address and IP
Type of Service found in the ...
... OSPF routes IP packets based solely on the destination IP address and IP
Type of Service found in the IP packet ...
... Type of Service found in the IP packet header. IP packets are routed
"as is" -- they are not encapsulated in any further protocol headers ...
... topology, lending the area protection
from bad routing data. An area is a generalization of an IP subnetted
network.
...
... destination and mask. Two different subnets
of the same IP network number may have different sizes (i.e., different
masks). This is commonly referred to as variable length subnets. A
...
... physical network to be assigned multiple IP network/subnet numbers.
We consider these to be separate networks ...
... networks are an exception - they are considered a single network no
matter how many (if any at all) IP network/subnet numbers are
assigned to them.
...
... range of IP addresses residing on a
single IP network/subnet. This specification displays network masks
...
... network mask for a class C
IP network is displayed as 0xffffff00. Such a mask is often
displayed elsewhere in the literature as 255.255.255.0.
...
... interface to a network has associated with it a
single IP address and mask (unless the network is an unnumbered
point-to-point ...
... point-to-point networks need not
be assigned IP addresses. Such a point-to-point network is called
...
...
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes ...
... address" in its external advertisements.
In the above example, Router RT7 would specify RTX's IP address as the
"forwarding address" for all those destinations ...
... TOS values are represented in OSPF exactly as they appear in the
IP packet header. This means that, for any destination, there can
...
... IP subnetting support ...
... variable-length subnet
masks. This means that a single IP class A, B, or C network number can
...
... subnets. The precise procedure for doing so is
beyond the scope of this specification. The specification however
establishes the following guideline: When an IP packet is forwarded, it
is always forwarded to the network that is the best match for the
...
... default route with destination of
0.0.0.0 and mask 0x00000000 is always a match for every IP destination.
Yet it is always less specific than any other match. Subnet ...
... Yet it is always less specific than any other match. Subnet masks must
be assigned so that the best match for any IP destination is
unambiguous.
...
... address
range: a class A, B, or C network number along with its natural IP mask.
Inside the area, any number of variable sized subnets could be defined.
...
... The OSPF protocol runs directly over IP, using IP protocol 89. OSPF
does not provide any explicit fragmentation/reassembly ...
... does not provide any explicit fragmentation/reassembly support. When
fragmentation is necessary, IP fragmentation/reassembly is used. OSPF
protocol packets have been designed so that large protocol packets can
...
... OSPF
protocol packets have been designed so that large protocol packets can
generally be split into several smaller protocol packets. This practice
is recommended; IP fragmentation should be avoided whenever possible.
...
... should be avoided whenever possible.
Routing protocol packets should always be sent with the IP TOS field set
to 0. If at all possible, routing protocol ...
... to 0. If at all possible, routing protocol packets should be given
preference over regular IP data traffic, both when being sent and
received. As an aid to accomplishing this, OSPF protocol ...
... received. As an aid to accomplishing this, OSPF protocol packets should
have their IP precedence field set to the value Internetwork Control
(see [RFC791]).
...
... routing packets (with the exception of Hellos)
are sent only over adjacencies. Note that this means that all protocol
packets travel a single IP hop, except those that are sent over virtual
adjacencies. The IP source address of an OSPF protocol ...
... packets travel a single IP hop, except those that are sent over virtual
adjacencies. The IP source address of an OSPF protocol packet is one
end of a router ...
... OSPF protocol packet is one
end of a router adjacency, and the IP destination address is either the
other end of the adjacency or an IP multicast ...
... to/subtracted from the timer interval at each firing.
IP multicast
Certain OSPF packets use IP multicast ...
... IP multicast
Certain OSPF packets use IP multicast. Support for receiving and
sending IP multicasts ...
... IP multicast. Support for receiving and
sending IP multicasts, along with the appropriate lower-level
protocol support, is required. These IP multicast packets never
...
... sending IP multicasts, along with the appropriate lower-level
protocol support, is required. These IP multicast packets never
travel more than one hop. For information on IP multicast, see
...
... protocol support, is required. These IP multicast packets never
travel more than one hop. For information on IP multicast, see
[RFC1112].
...
... All OSPF implementations must be able to calculate separate routes
based on IP Type of Service. However, to save routing table space
...
... router in the AS. One
possible implementation strategy would be to use the smallest IP
interface address belonging to the router.
...
... backbone. If assigning subnetted networks as separate areas,
the IP network number could be used as the Area ID.
...
... address ranges are not allowed to overlap). As an
example, if an IP subnetted network is to be its own separate OSPF
...
... area, the area is defined to consist of a single address range - an
IP network number with its natural (class A, B or C) mask.
...
... router should
advertise into the area. There can be a separate cost configured
for each IP TOS. See Section 12.4.3 for more information.
...
... network. The Link State ID for this advertisement (see Section
12.1.4) is the IP interface address of the Designated Router. The
...
... address of the Designated Router. The
IP network number can then be obtained by using the subnet/network
...
...
network nodes are actually labelled with the IP address of their
Designated Router. It follows that when the Designated Router changes,
...
... Routing protocol packets are sent along adjacencies only (with the
exception of Hello packets, which are used to discover the adjacencies).
This means that all protocol packets travel a single IP hop, except
those sent over virtual links.
...
... per-interface basis).
The IP destination address for the packet is selected as follows. On
physical ...
... OSPF packets are sent as unicasts, i.e.,
sent directly to the other end of the adjacency. In this case, the IP
destination is just the neighbor ...
...
destination is just the neighbor IP address associated with the other
end of the adjacency (see Section 10). The only packets not sent as
unicasts are on ...
...
The IP source address should be set to the IP address of the sending
interface. Interfaces to unnumbered ...
... point-to-point networks have no
associated IP address. On these interfaces, the IP source should be set
...
... associated IP address. On these interfaces, the IP source should be set
to any of the other IP addresses belonging to the router ...
... interfaces, the IP source should be set
to any of the other IP addresses belonging to the router. For this
reason, there must be at least one IP address ...
... IP addresses belonging to the router. For this
reason, there must be at least one IP address assigned to the router.[2]
...
... networks. However, each virtual link does
have an interface IP address (discovered during the routing table build
process) which is used as the IP ...
... interface IP address (discovered during the routing table build
process) which is used as the IP source when sending packets over the
virtual link ...
... 3]
In order for the packet to be accepted at the IP level, it must pass a
number of tests, even before the packet is passed to OSPF for
...
... processing:
o The IP checksum must be correct.
o The packet's IP ...
... IP checksum must be correct.
o The packet's IP destination address must be the IP address of the
...
... AllSPFRouters or AllDRouters.
o The IP protocol specified must be OSPF (89).
...
... o Locally originated packets should not be passed on to OSPF. That
is, the source IP address should be examined to make sure this is
not a multicast packet that the router ...
... interface. In this case, the
packet has been sent over a single hop. Therefore, the packet's
IP source address must be on the same network as the receiving
...
... receiving
interface. This can be determined by comparing the packet's IP
source address to the interface's IP address, after masking both
...
... interface. This can be determined by comparing the packet's IP
source address to the interface's IP address, after masking both
addresses with the interface ...
... backbone area).
o Packets whose IP destination is AllDRouters should only be accepted
if the state ...
... non-broadcast)
the sender is identified by the IP source address found in the packet's
IP header. If the receiving ...
... the sender is identified by the IP source address found in the packet's
IP header. If the receiving interface is a point-to-point link ...
... network;
each interface structure has at most one IP interface address (see
below). The support for multiple addresses ...
... IP interface address
The IP address associated with the interface. This appears as the
IP source address ...
... IP address associated with the interface. This appears as the
IP source address in all routing protocol packets originated over
this interface ...
... not have an associated IP address.
IP interface mask
This indicates the portion of the IP interface address ...
...
IP interface mask
This indicates the portion of the IP interface address that
identifies the attached network ...
... network. This is often referred to as the
subnet mask. Masking the IP interface address with this value
yields the IP network ...
... Designated Router: its Router ID and its interface IP address on the
network. The Designated Router ...
... link state advertisement is labelled with the
Designated Router's IP address. This item is initialized to 0,
which indicates the lack of a Designated Router.
...
... router links advertisement. There may be a
separate cost for each IP Type of Service. The cost of an interface
...
... interface or
through something like a bit error test. For this reason, IP
packets may still be addressed to an interface in Loopback state ...
... routers
attached to a common network. The Hello packet also contains the IP
address mask of the attached network (Network Mask). On unnumbered
...
... Options field should be
set if the router is capable of calculating separate routes for each IP
TOS. The E-bit should be set if and only if the attached area is
...
... point-to-point networks, Hello
packets are sent every HelloInterval seconds to the IP multicast address
AllSPFRouters. On virtual links ...
... 's OSPF router ID or by its
Neighbor IP address (see below). Thus if the OSPF router and another
router have multiple attached ...
... interface to the attached
network. Used as the Destination IP address when protocol packets
are sent as unicasts along this adjacency. Also used in router ...
... Designated Router (see Section
12.4.1). The neighbor IP address is learned when Hello packets are
received from the neighbor. For virtual links ...
... neighbor. For virtual links, the neighbor IP
address is learned during the routing table build process (see
Section 15).
...
... OSPF packets will have checked the validity
of the IP header and the OSPF packet header. Next, the values of the
...
... broadcast or non-broadcast)
the source is identified by the IP source address found in the Hello's
IP header. If the receiving ...
... )
the source is identified by the IP source address found in the Hello's
IP header. If the receiving interface is a point-to-point link ...
... structure's Neighbor IP address to the packet's
IP source address.
Now the rest of the Hello Packet is examined, generating events to be
...
... (Designated Router field = neighbor IP address) and the Backup
Designated Router field in the packet is equal to 0.0.0.0 and the
...
... Designated Router (Backup Designated Router field = neighbor IP
address) and the receiving interface is in state ...
... bit should be set if and only if the router is capable of
calculating separate routes for each IP TOS. The E-bit should be set if
...
... routing table data structure contains all the information necessary
to forward an IP data packet toward its destination. Each routing table
...
...
entry describes the collection of best paths to a particular
destination. When forwarding an IP data packet, the routing table entry
providing the best match for the packet's IP ...
... IP data packet, the routing table entry
providing the best match for the packet's IP destination is located.
...
... Destination ID = DefaultDestination). When the default route
exists, it matches all IP destinations (although any other matching
entry is a better match). Finding the routing table ...
... entry is a better match). Finding the routing table entry that best
matches an IP destination is further described in Section 11.1.
...
... destination can be one of three types. Only the first type,
Network, is actually used when forwarding IP data traffic. The
other destinations ...
... A range of IP addresses, to which IP data traffic may be
forwarded. This includes IP networks ...
... IP data traffic may be
forwarded. This includes IP networks (class A, B, or C), IP
subnets, and single IP ...
... forwarded. This includes IP networks (class A, B, or C), IP
subnets, and single IP hosts. The default route ...
... IP networks (class A, B, or C), IP
subnets, and single IP hosts. The default route also falls in
...
... networks, the identifier is their
associated IP address. For all other types, the identifier is the
OSPF Router ...
... Only defined for networks. The network's IP address together with
its address mask defines a range ...
... address mask defines a range of IP addresses. For IP subnets,
the address mask is referred to as the subnet ...
... capabilities currently defined by this specification are the ability
to route based on IP TOS and the ability to process AS external
...
...
The set of paths to use for a destination may vary based on IP Type of
Service and the OSPF area to which the paths belong. This means that
...
...
Type of Service
There can be a separate set of routes for each IP Type of Service.
The encoding ...
... networks, the next hop also includes
the IP address of the next router (if any) in the path towards the
destination ...
...
When an IP data packet is received, an OSPF router finds the routing
table entry that best matches the packet's destination ...
... routing table entry (i.e., all existing routing table entries are pruned
for some reason or another). In this case, the packet's IP destination
is considered unreachable. Instead of being forwarded, the packet should
...
... routing table entry describes a (set of)
path(s) to a range of IP addresses. If the data packet's IP
...
... destination falls into an entry's range of IP addresses, the routing
table entry is called a match. (It is quite likely that multiple
entries will match the data packet ...
... will match all packets.)
(2) Suppose that the packet's IP destination falls into one of the
router ...
... networks contained
in the area address range. The packet's IP destination is then
required to belong to one of these constituent networks ...
... (most specific) match. Another way of saying this is to choose the
remaining entry that specifies the narrowest range of IP
addresses.[10] For example, the entry for the address/mask pair of
...
... remaining. Each routing entry will specify the same range of IP
addresses, but a different IP Type of Service. Select the routing
table ...
... routing entry will specify the same range of IP
addresses, but a different IP Type of Service. Select the routing
table entry whose TOS ...
... links advertisement if and only if the router is capable
of calculating separate routes for each IP TOS (see Section 2.4). The
T-bit ...
... network, the Link
State ID is either the network's IP address (as in type 3 summary link
advertisements and in AS ...
... external link advertisements) or the network's
IP address is easily derivable from the Link State ID (note that masking
a network ...
... subnet
mask yields the network's IP address). When the link state
advertisement is describing a router, the Link State ...
... 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 TOS. TOS is encoded in an OSPF link ...
... links advertisements have LS type = 1. The router indicates whether it
is willing to calculate separate routes for each IP TOS by setting (or
resetting) the T-bit ...
... routers
and transit networks, this field specifies the IP interface address of
the associated router interface ...
... networks, this field
specifies the network's IP address mask.
Finally, the cost of using the link ...
... router, and the Link Data should specify the
interface IP address.
- If this is a numbered point-to-point ...
... network) whose Link ID is the neighbor's IP address, whose
Link Data is the mask 0xffffffff indicating a host ...
... interface to an unnumbered
serial line. The Link ID should be set to the IP interface address,
the Link ...
... (stub network) whose Link ID is the IP network number of the
attached network and whose Link ...
... router
itself) and whose Link Data is the interface IP address. Otherwise,
add a link as if the interface ...
... links are functional, and that Router IDs are assigned as the
smallest IP interface address.
...
... 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 (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
indicated that it is capable of calculating separate routes based on IP
...
... Designated
Router RT4's IP interface to 192.1.1.0). Note also that RT3 has
indicated that it is capable of calculating separate routes based on IP
TOS, through setting the T-bit in the ...
... Designated Router
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
...
... Link State ID for a network links advertisement is the IP interface
address of the Designated Router. This value, masked by the ...
... link advertisements originated by router RT4 follow.
The actual IP addresses for the networks and routers in question have
...
... Summary link advertisements pertain to a single destination (IP network
or AS boundary router ...
... metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange EGP ...
... Update packets are multicast.
The destination IP address specified for the Link State Update
...
... neighbor (i.e.,
those in state Exchange or greater). The destination IP addresses
for these packets are the neighbors' IP addresses ...
... accomplished by sending the delayed Link State Acknowledgment packets as
multicasts. The Destination IP address used depends on the state of the
interface. If the ...
... routing table build process (through construction of the routing
table entry for the other endpoint), so are the IP interface address
for the virtual interface ...
... for the virtual interface and the virtual neighbor's IP address.
These are used when sending protocol packets over the virtual link.
...
... Link Data set to
the virtual interface's IP address. See Section 12.4.1 for more
information. Also, it may be the case that there is a TOS 0 path,
...
... OSPF Router ID. For network vertices, this is
the IP address of the network's Designated Router.
...
... networks, the next hop also includes
the IP address of the next router (if any) in the path towards the
destination ...
... network is located. The entry's destination ID is the
IP network number, which can be obtained by masking the Vertex ID
(Link State ID) with its associated subnet ...
... destination installed in the routing table), multiple vertices have
mapped to the same IP network. For example, this can occur when a
new Designated Router is being established. In this case, the
...
... network (the usual case),
a routing table entry for the IP network should be added. The
routing table entry's Link State ...
... . The outgoing
interface to use can then be derived from the next hop IP address (or it
can be inherited from the parent network).
...
... considered in turn. Most AS external advertisements describe routes to
specific IP destinations. An AS external advertisement can also
...
... destination N. Examine the forwarding address specified in the
external advertisement. This indicates the IP address to which
packets for the destination should be forwarded. If forwarding
...
... link, which will cause a virtual adjacency to begin to form
(see Section 10.3). At this time the virtual interface's IP address
and the virtual neighbor's IP address ...
... IP address
and the virtual neighbor's IP address are also calculated.
If the entry indicates that the area border router ...
...
The OSPF protocol can calculate a different set of routes for each IP
TOS (see Section 2.4). Support for TOS-based ...
... data
traffic, regardless of the TOS indications in the data packet's IP
header. A router that does not support TOS indicates this fact to the
...
... router's interfaces to be unnumbered point-to-point links. In this case, an IP address must be assigned to the router. This address will then be advertised in the router ...
... Note that in these cases both interfaces, the non-virtual and the virtual, would have the same IP address. ...
... Note that no host route is generated for, and no IP packets can be addressed to, interfaces to unnumbered point-to-point networks ...
... The address space of IP networks and the address space of OSPF Router IDs may overlap. That is, a network ...
... address space of OSPF Router IDs may overlap. That is, a network may have an IP address which is identical (when considered as a 32-bit number) to some router's Router ID ...
... OSPF
link state advertisements. The OSPF protocol runs directly over the IP
network layer. Before any data formats are described, the details of
...
... OSPF does not define a way to fragment its protocol packets, and depends
on IP fragmentation when transmitting packets larger than the network ...
... , 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).
The other important features of OSPF ...
... over multi-access networks. Two distinct IP multicast addresses are
used. Packets destined to these multicast addresses ...
... multicast addresses should never be
forwarded. Such packets are meant to travel a single hop only. To
ensure that these packets will not travel multiple hops, their IP
TTL must be set to 1.
AllSPFRouters
...
...
o OSPF is IP protocol number 89. This number has been registered with
the Network Information Center. IP protocol number ...
... IP protocol number 89. This number has been registered with
the Network Information Center. IP protocol number assignments are
documented in [RFC1060].
...
... TOS. However, all OSPF routing
protocol packets are sent with the DTR bits in the IP header's TOS
field (see [RFC791 ...
...
o Routing protocol packets are sent with IP precedence set to
Internetwork Control. OSPF protocol packets should be given
...
... Internetwork Control. OSPF protocol packets should be given
precedence over regular IP data traffic, in both sending and
receiving ...
... traffic, in both sending and
receiving. Setting the IP precedence field in the IP header to
Internetwork Control [RFC791 ...
... receiving. Setting the IP precedence field in the IP header to
Internetwork Control [RFC791] may help implement this objective.
...
...
Checksum
The standard IP checksum of the entire contents of the packet,
excluding the 64-bit authentication ...
... router. The Designated Router is identified here
by its IP interface address on the network. Set to 0 if there is no
...
... router. The Backup Designated Router is
identified here by its IP interface address on the network. Set to
...
... Designated Router (from which the network's
IP address can be derived). The Link State ID is further discussed
in Section 12.1.4.
...
... field if and only if the router is able to calculate a separate set of
routes for each IP TOS. Router links ...
... networks
this field specifies the network's IP address mask. For the other link
types the Link Data specifies the router ...
... network mask. For the other link
types it specifies the router's associated IP interface address.
This latter piece of information is needed during the routing table ...
... This latter piece of information is needed during the routing table
build process, when calculating the IP address of the next hop. See
Section 16.1.1 for more details.
...
... 8 when both are specified.
TOS IP type of service that this metric refers to. The encoding of TOS ...
... The advertisement's Link State ID field lists the IP interface address
of the Designated Router ...
... Type 3 link state advertisements are used when the destination is an IP
network. In this case the advertisement's Link State ID field is an IP
network number. When the destination ...
... destination is an IP
network. In this case the advertisement's Link State ID field is an IP
network number. When the destination is an AS boundary router ...
... link state advertisements, this indicates the
destination's IP network mask. For example, when advertising the
location of a class A network ...
... destination. For these advertisements the Link State ID field specifies
an IP network number. AS external link advertisements are also used to
...
... Network Mask is set to 0.0.0.0.
Separate costs may be advertised for each IP Type of Service. The
encoding of ...
...
Network Mask
The IP network mask for the advertised destination. For example,
when advertising a class ...
... advertisements and in type 3 summary link advertisements for stub
areas. Its value is the IP address 0.0.0.0.
...
... attached to a network must agree on that
network's IP network number and mask.
Some parameters may be determined by router ...
... algorithm for Router ID assignment is to
choose the largest or smallest IP address assigned to the router.
If a router ...
... backbone. If the area represents a subnetted
network, the IP network number of the subnetted network may be used
for the area ID ...
... ranges
An OSPF area is defined as a list of [IP address, mask] pairs. Each
pair describes a range of IP addresses ...
... IP address, mask] pairs. Each
pair describes a range of IP addresses. Networks and hosts are
...
... area. The area would be configured as a single address range, whose
IP address is the address of the subnetted network, and whose mask
...
... router should
advertise into the area. There can be a separate cost configured
for each IP TOS. See Section 12.4.3 for more information.
...
...
Some of the configurable router interface parameters (such as IP
interface address and subnet mask) actually imply properties of the
...
... identifies the router over the entire internet. An IP address is
not required on serial lines. Such a serial line is called
"unnumbered".
...
... not required on serial lines. Such a serial line is called
"unnumbered".
IP interface mask
This denotes the portion of the IP interface address ...
...
IP interface mask
This denotes the portion of the IP interface address that
identifies the attached network ...
... router links advertisement. There may be
a separate cost for each IP Type of Service. The interface output
...
... link acts like an unnumbered point-
to-point link, it does have an associated IP interface address. This
address is used as the ...
... address. This
address is used as the IP source in protocol packets it sends along the
virtual link, and is set dynamically during the routing table ...
... point-to-point networks, looped router interfaces, or IP hosts that
are directly connected to the router ...
... host, in terms of the link state
metric. There may be multiple costs configured, one for each IP
TOS. However, since the host ...
... link
state advertisements to be originated. The logging message produced
should include the interface's IP address (or other name), interface
type (virtual link, etc.) and old and new state ...
... documented in Section 10.3. The logging message produced should
include the neighbor IP address, and old and new state values.
...
... networks. See Section 9.4. The logging message produced should
include the network IP address, and the old and new (Backup)
Designated Routers.
...
... OSPF neighbor. The
logging message produced should include the IP source and
destination addresses, the router ID ...
... authentication, and that Area 1 is not using any authentication.
The number of nets includes IP networks, subnets, and hosts (this is
...
... address to which packets should be forwarded (Section 12.4.4). In the
above example, router X will put Y's IP address into this field. If the
field is 0, packets are (as before) forwarded to the originator of the
advertisement. A different forwarding address ...
... of the OSPF specification prohibits the collapsing of multiple
backbone IP networks/subnets into a single summary link. This
...
... When forwarding an IP data packet, a router looks up the packet's IP
destination in the routing table. This determines the packet's ...