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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
Figure 5-21. RIP Message Format
RIP version 2 also adds a 16-bit route tag that is retained and sent with router updates. It can be used
to identify the origin of the route.
Because the version number in RIP2 occupies the same octet as in RIP1, both versions of the protocols
can be used on a given router simultaneously without interference.
Open Shortest Path First (OSPF)
The Open Shortest Path First (OSPF) routing protocol uses a link-state algorithm to determine routes to
network destinations. A “link” is an interface on a router and the “state” is a description of that
interface and its relationship to neighboring routers. The state contains information such as the IP
address, subnet mask, type of network the interface is attached to, other routers attached to the
network, etc. The collection of link-states is then collected in a link-state database that is maintained
by routers running OSPF.
OSPF specifies how routers will communicate to maintain their link-state database and defines several
concepts about the topology of networks that use OSPF.
To limit the extent of link-state update traffic between routers, OSPF defines the concept of Area. All
routers within an area share the exact same link-state database, and a change to this database on one
router triggers an update to the link-state database of all other routers in that area. Routers that have
interfaces connected to more than one area are called Border Routers and take the responsibility of
distributing routing information between areas.
One area is defined as Area 0 or the Backbone. This area is central to the rest of the network in that all
other areas have a connection (through a router) to the backbone. Only routers have connections to the
backbone and OSPF is structured such that routing information changes in other areas will be
introduced into the backbone, and then propagated to the rest of the network.
When constructing a network to use OSPF, it is generally advisable to begin with the backbone (area 0)
and work outward.
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