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Distance vector versus link-state

By stretch | Thursday, October 2, 2008 at 3:36 a.m. UTC

There are two major classes of routing protocol: distance vector and link-state. It's easy to remember which protocols belong to either class, but comprehending the differences between the two classes takes a bit more effort. Distance vector routing is so named because it involves two factors: the distance, or metric, of a destination, and the vector, or direction to take to get there. Routing information is only exchanged between directly connected neighbors. This means a router knows from which neighbor a route was learned, but it does not know where that neighbor learned the route; a router can't see beyond its own neighbors. This aspect of distance vector routing is sometimes referred to as "routing by rumor." Measures like split horizon and poison reverse are employed to avoid routing loops. Link-state routing, in contrast, requires that all routers know about the paths reachable by all other routers in the network. Link-state information is flooded throughout the link-state domain (an area in OSPF or IS-IS) to ensure all routers posess a synchronized copy of the area's link-state database. From this common database, each router constructs its own relative shortest-path tree, with itself as the root, for all known routes. Consider the following topology.

Both distance vector and link-state routing protocols are suitable for deployment on this network, but each will go about propagating routes in a different manner. Distance Vector If we were to run a distance vector routing protocol like RIP or EIGRP on this topology, here's how R1 would see the network, assuming each link has a metric of 1 (locally connected routes have been omitted):

http://packetlife.net/blog/2008/oct/2/distance-vector-versus-link-state/

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Notice that although R1 has connectivity to all subnets, it has no knowledge of the network's structure beyond its own links. R4 has even less...