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IP
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... SPF technology.
This is a departure from the Bellman-Ford base used by traditional
TCP/IP internet routing protocols.
...
... Internet Engineering Task Force. It has been designed expressly for
the TCP/IP internet environment, including explicit support for IP
...
... the TCP/IP internet environment, including explicit support for IP
subnetting, TOS-based routing ...
... authentication of
routing updates, and utilizes IP multicast when sending/receiving
the updates. In addition, much work has been done to produce a
...
... OSPF routes IP packets based solely on the destination IP
address and IP Type of Service found in the IP packet ...
... OSPF routes IP packets based solely on the destination IP
address and IP Type of Service found in the IP packet header ...
... 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 subnetting. A packet is routed
...
... for one physical network to be assigned multiple IP
network/subnet numbers. We consider these to be separate
networks ...
... - 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/supernet. This
specification displays network ...
... For example, the 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 ...
... hiding/protection/reduction. Finally, the algorithm has been
modified for efficient operation in TCP/IP internets.
...
...
Each network (stub or transit) in the graph has an IP address and
associated network mask. The mask indicates the number of nodes ...
... external advertisements. In the above example, Router RT7 would
specify RTX's IP address as the "forwarding address" for all
those destinations ...
...
OSPF can calculate a separate set of routes for each IP Type of
Service. This means that, for any destination, there can
...
... destination, there can
potentially be multiple routing table entries, one for each IP
TOS. The IP ...
... IP subnetting support ...
... destination enables the
implementation of what is commonly referred to as variable-
length subnetting. This means that a single IP class A, B, or C
network ...
... doing so is beyond the scope of this specification. This
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 packet's destination ...
... 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 ...
... 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.
...
... 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. External to the area,
...
... The OSPF protocol runs directly over IP, using IP protocol 89.
OSPF does not provide any explicit fragmentation/reassembly ...
... fragmentation/reassembly
support. When fragmentation is necessary, IP
fragmentation/reassembly is used. OSPF protocol ...
... 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.
...
...
Routing protocol packets should always be sent with the IP TOS
field set to 0. If at all possible, routing protocol ...
... field set 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,
...
... when being sent and received. As an aid to accomplishing this,
OSPF protocol packets should have their IP precedence field set
to the value Internetwork Control (see [RFC791]).
...
... Hellos) are sent only over adjacencies. Note that this means
that all OSPF protocol packets travel a single IP hop, except
those that are sent over virtual adjacencies. The IP source
address of an OSPF protocol ...
... 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 address.
...
... timer interval at each firing.
IP multicast
Certain OSPF packets take the form of IP multicast ...
... IP multicast
Certain OSPF packets take the form of IP multicast
datagrams. Support for receiving ...
... datagrams. Support for receiving and sending IP multicast
datagrams, along with the appropriate lower-level protocol
...
... datagrams, along with the appropriate lower-level protocol
support, is required. The IP multicast datagrams used by
OSPF ...
... OSPF never travel more than one hop. For this reason, the
ability to forward IP multicast datagrams is not required.
For information on IP multicast ...
... subnet support
The router's IP protocol support must include the ability to
divide a single IP class ...
... router's IP protocol support must include the ability to
divide a single IP class A, B, or C network number into many
...
... variable-length subnetting; see Section 3.5 for details.
IP supernetting support
The router's IP protocol ...
... IP supernetting support
The router's IP protocol support must include the ability to
aggregate contiguous collections of IP class ...
... router's IP protocol support must include the ability to
aggregate contiguous collections of IP class A, B, and C
networks ...
... networks into larger quantities called supernets.
Supernetting has been proposed as one way to improve the
scaling of IP routing in the worldwide Internet. For more
information on IP ...
... IP routing in the worldwide Internet. For more
information on IP supernetting, see [RFC1519].
...
... All OSPF implementations must be able to calculate separate
routes based on IP Type of Service. However, to save
routing table ...
... AS.
One possible implementation strategy would be to use the
smallest IP interface address belonging to the router. If a
...
... 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 ...
... 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 IP network ...
... IP
interface 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 ...
... adjacencies). This means that all routing protocol packets travel a
single IP hop, except those sent over virtual links.
...
... 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 ...
... 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
...
... belonging to the router. For this reason, there must be at
least one IP address assigned to the router.[2] Note that, for
...
... networks. However, each virtual link
does have an IP interface address (discovered during the routing
table build process) which is used as the IP ...
... IP interface 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 ...
... of the receiving interface, or one of the IP multicast
addresses AllSPFRouters or AllDRouters.
...
... 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 ...
... IP source address
to the interface's IP address, after masking both
addresses with the interface ...
... non-broadcast) the sender is identified by
the IP source address found in the packet's IP header. If the
receiving ...
... sender is identified by
the IP source address found in the packet's IP header. If the
receiving interface ...
... 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 ...
... IP address.
IP interface mask
Also referred to as the subnet mask, this indicates the portion
...
... Also referred to as the subnet mask, this indicates the portion
of the IP interface address that identifies the attached
network ...
... address that identifies the attached
network. Masking the IP interface address with the IP interface
...
... network. Masking the IP interface address with the IP interface
mask yields the IP network number of the attached network ...
... address with the IP interface
mask yields the IP network number of the attached network. On
point-to-point ...
... point-to-point networks and virtual links, the IP interface mask
is not defined. On these networks, the link ...
... networks, the link itself is not
assigned an IP network number, and so the addresses of each side
of the link ...
... link state advertisement is
labelled with the Designated Router's IP address. The
Designated Router is initialized to 0.0.0.0, which indicates the
...
... router links advertisement. There may be
a separate cost for each IP Type of Service. The cost of an
interface ...
... routers attached to
a common network. The Hello packet also contains the IP address
mask of the attached network (Network ...
... Options field should be set if the router is capable of
calculating separate routes for each IP TOS. The E-bit should
...
... point-to-point networks,
Hello packets are sent every HelloInterval seconds to the IP
multicast address AllSPFRouters. On virtual links, Hello
...
... OSPF Router ID
or by its Neighbor IP address (see below). Thus if the OSPF router
and another router ...
... interface to the
attached network. Used as the Destination IP address when
protocol packets are sent as unicasts along this adjacency.
...
... Designated Router (see Section 12.4.1). The Neighbor 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 build process (see Section 15).
...
... broadcast or non-broadcast) the source is identified by the IP
source address found in the Hello's IP header. If the receiving
...
... broadcast or non-broadcast) the source is identified by the IP
source address found in the Hello's IP header. If the receiving
interface ...
... structure's Neighbor IP address to the packet's IP source
address.
Now the rest of the Hello Packet is examined, generating events
...
... Designated Router (Designated Router field = Neighbor IP
address) and the Backup Designated Router field in the
packet is equal to 0.0.0.0 and the receiving ...
... bit should be set if and only if the router is capable of
calculating separate routes for each IP TOS. The E-bit should
...
... routing table data structure contains all the information
necessary to forward an IP data packet toward its destination. Each
routing table ...
... 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. The matching routing table ...
... DefaultDestination, Address Mask = 0x00000000). When the default
route exists, it matches all IP destinations (although any other
matching entry is a better match). Finding the routing table ...
... matching 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, IP ...
... forwarded. This includes IP networks (class A, B, or C), IP
subnets, IP supernets and single IP hosts ...
... IP networks (class A, B, or C), IP
subnets, IP supernets and single IP hosts. The default
route ...
... class A, B, or C), IP
subnets, IP supernets and single IP hosts. The default
route also falls in this category.
...
... 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 ...
... with its address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet ...
... address mask defines a range of IP addresses. For IP
subnets, the address mask is referred to as the subnet mask.
...
... optional capabilities currently defined by this specification
are the ability to route based on IP TOS and the ability to
process AS ...
...
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
...
... Type of Service
There can be a separate set of routes for each IP Type of
Service. The encoding of TOS ...
... networks, the next hop also
includes the IP address of the next router (if any) in the path
towards the destination ...
... 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 be dropped and an ICMP
destination ...
... routing table entry describes
a (set of) path(s) to a range of IP addresses. If the data
packet's IP destination ...
... range of IP addresses. If the data
packet's IP destination falls into an entry's range of IP
addresses ...
... 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 ...
... networks contained in the area address range.
The packet's IP destination is then required to belong to
one of these constituent networks ...
... to choose the remaining entry that specifies the narrowest
range of IP addresses.[10] For example, the entry for the
address ...
... routing entry will specify the same
range of IP addresses, but a different IP Type of Service.
...
... only if the router is capable of calculating separate routes
for each IP TOS (see Section 2.4). The T-bit should always
...
... network
(LS type = 2, 3 or 5), the network's IP address is easily
derived by masking the Link State ID with the network ...
... 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. The encoding of TOS ...
... ....................................
Figure 15: Area 1 with IP addresses shown
The format of a router ...
... router
indicates whether it is willing to calculate separate routes
for each IP TOS by setting (or resetting) the T-bit of the
...
... routers and
virtual links, this field specifies the IP interface address
of the associated router interface ...
... to stub networks, this field specifies the network's IP
address mask. For unnumbered point-to-point networks, the
...
... point networks, the Link Data should specify the IP
interface address. For unnumbered point-to-point
...
... point network) and the neighboring router's IP
address is known, add a Type 3 link (stub 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 Data
...
... link (stub network) whose Link ID is the IP
network number of the attached network and whose Link
...
... network) whose Link ID is the host's IP address
and whose Link Data is the mask of all ones (0xffffffff).
...
... 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 ...
... 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 ...
... 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 ...
... links = 2
Link ID = 192.1.1.4 ;IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface ...
... IP address of Desig. Rtr.
Link Data = 192.1.1.3 ;RT3's IP interface to net
Type = 2 ;connects to transit network
...
... network links advertisement is the
IP interface address of the Designated Router. This value,
...
... Link State ID
equal to one of the router's IP interface addresses and
their Advertising Router ...
... links
Link State ID = 192.1.1.4 ;IP address of Desig. Rtr.
Advertising Router = 192.1.1.4 ;RT4's Router ID ...
... link advertisements originated
by Router RT4 follow. The actual IP addresses for the
networks and routers ...
... link advertisements pertain to a single destination
(IP network or AS boundary router). However, for a single
...
... Link State ID is set to the destination network's IP address
(if necessary, the Link State ID can also have one or more
...
... metric = 1
Forwarding address = RTX's IP address
In figure 16, suppose instead that both RTA and RTB exchange
...
... 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 ...
... Link State
ID is one of the router's own IP interface addresses but whose
Advertising Router ...
... Link State Acknowledgment
packets as multicasts. The Destination IP address used depends
on the state of the interface ...
... routing table entry for the other endpoint), so are the IP
interface address for the virtual interface and the virtual
...
... interface and the virtual
neighbor's IP address. These are used when sending OSPF
protocol packets over the virtual link. Note that when one (or
...
... point-to-point link, it may be impossible to
calculate either the virtual interface's IP address and/or the
virtual neighbor's IP address ...
... IP address and/or the
virtual neighbor's IP address, thereby causing the virtual link
to fail.
...
... Link
Data is set to the virtual interface's IP address. See Section
12.4.1 for more information. Note that it may be the case that
there is a TOS ...
... 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 ...
... virtual links that uses Area A as its
Transit area: the virtual link is declared up, the IP
address of the virtual interface is set to the IP address of
...
... link is declared up, the IP
address of the virtual interface is set to the IP address of
the outgoing interface calculated above for ABR ...
... 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
...
... 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
...
... network (the
usual case), a routing table entry for the IP network should
be added. The routing table entry's Link State ...
... interface to use
can then be derived from the next hop IP address (or it can
be inherited from the parent network).
...
... the AS external link advertisement. This indicates the IP
address to which packets for the destination should be
forwarded. If the forwarding address ...
... begin to form (see Section 10.3). At this time the virtual
link's IP interface address and the virtual neighbor's
...
... neighbor's
Neighbor 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 routing ...
... 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 ...
... Deering, S., "Host extensions for IP multicasting", STD 5, RFC 1112std5, Stanford University, May 1988. ...
... McCloghrie, K., and M. Rose, "Management Information Base for network management of TCP/IP-based internets: MIB-II", STD 17, RFC 1213std17 ...
... state advertisements. The OSPF protocol runs directly over the
IP network layer. Before any data formats are described, the
...
... OSPF does not define a way to fragment its protocol packets, and
depends on IP fragmentation when transmitting packets larger than
...
... 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
...
... 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 ...
... sent over multi-access networks. Two distinct IP multicast
addresses are used. Packets sent to these multicast addresses ...
... should never be forwarded; they 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 [RFC1340].
...
...
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
...
... Checksum
The standard IP checksum of the entire contents of the packet,
starting with the OSPF packet ...
... router. The Designated Router is
identified here by its IP interface address on the network. Set
...
... in the view of the advertising router. The Backup Designated
Router is identified here by its IP interface address on the
network ...
... links advertisements the Link State ID is set to the
IP interface address of the network's Designated Router ...
... Designated Router (from
which the network's IP address can be derived). The Link State
ID is further discussed in Section 12.1.4.
...
... advertisement's Option field if and only if the router is able to
calculate a separate set of routes for each IP TOS. Router links ...
... to stub networks, it specifies the network's IP address mask.
For unnumbered point-to-point connections, it specifies the
...
... ifIndex value. 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 ...
... address.
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.
...
... 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 ...
... IP network. In this case the advertisement's Link State ID field is
an IP network number (if necessary, the Link State ID can also have
one or more of the network ...
... destination network's IP address 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 (if necessary, the Link State
ID can also have one or more of the network ...
... Network Mask
The IP address mask for the advertised destination. For
example, when advertising a class ...
... link advertisements and in stub areas' type 3 summary link
advertisements. Its value is the IP address 0.0.0.0.
...
... 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 ...
... address range consists of the following items:
[IP address, mask]
Describes the collection of IP addresses contained
...
... [IP address, mask]
Describes the collection of IP addresses contained
in the address range. Networks ...
... other areas. Status is set to Advertise by default.
As an example, suppose an IP subnetted network is to be its
own OSPF ...
... OSPF area. The area would be configured as a single
address range, whose IP address is the address of the
subnetted network ...
... link that the 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
...
... identifies the router over the entire internet. An IP
address is not required on serial lines. Such a serial line
is called "unnumbered".
...
... is called "unnumbered".
IP interface mask
Also referred to as the subnet mask, this indicates the
...
... Also referred to as the subnet mask, this indicates the
portion of the IP interface address that identifies the
attached network ...
... address that identifies the
attached network. Masking the IP interface address with the
IP interface ...
... IP interface address with the
IP interface mask yields the IP network number of the
attached network ...
... address with the
IP interface mask yields the IP network number of the
attached network. On point-to-point ...
... point-to-point networks and virtual
links, the IP interface mask is not defined. On these
networks, the link ...
... networks, the link itself is not assigned an IP network
number, and so the addresses of each side of the link ...
... router links
advertisement. There may be a separate cost for each IP
Type of Service. The interface ...
... acts like an unnumbered point-to-point link, it does have an
associated IP interface address. This address is used as the IP ...
... IP interface address. This address is used as the IP
source in OSPF protocol packets it sends along the virtual link ...
... networks, looped router interfaces,
or IP hosts that are directly connected to the router (e.g., via
...
... link state metric. There may be multiple costs configured,
one for each IP TOS. However, since the host probably has
...
... link
advertisements is set equal to the described network's IP
address. In the above example, RFC 1247(-> 1583(-> 2178(-> 2328std54))) would assign both
advertisements the Link State ...
... whose Link State ID is equal to one of the router's own IP
interface addresses should be considered to be self-originated,
regardless of the setting of the advertisement's Advertising
...
... and summary link advertisements is set to the described network's IP
address. This memo relaxes that requirement, allowing one or more of
the network ...
... algorithms used is
that the network's IP address should be used as the Link State ID
(the RFC 1247(-> 1583(-> 2178(-> 2328std54))) ...