Most routing protocols are designed around one of the following two routing
methods: distance vector or link-state.
Distance vector routing: In distance vector routing, a router does not have to know the
entire path to every network segment; the router only has to know the direction, or vector,
in which to send the packet. The distance vector routing approach determines the direction
(vector) and distance (hop count) to any network in the internetwork. Distance vector
algorithms periodically (such as every 30 seconds by default for Routing Information
Protocol [RIP]) send all or portions of their routing table to their adjacent neighbors.
Routers running a distance vector routing protocol will send periodic updates, even if there
are no changes in the network. By receiving the routing table of a neighbor, a router can
verify all the known routes and make changes to its local routing table based on updated
information received from the neighboring router. This process is also known as ???routing
by rumor,??? because the understanding that a router has of the network topology is based on
the perspective of the routing table of a neighbor router. Figure 4-5 shows how distance
vector protocols determine routes.
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