Distance-Vector Routing Basics

Distance-vector routing functions by passing routing tables between devices on the network. So the first router looks at its connected interfaces, builds a routing table, and passes that information to other routers on its connected interfaces.

If the router receives routing tables from other routers, it updates any metrics used for choosing routes, such as RIP’s hop count, and adds that information to its routing table. This information is then sent out with the routing table updates that are sent to neighboring routers.

Now, this process sounds really simple — the routers will just pass that information out and then everyone will know how to get to all other locations on the network — but there are some issues that affect most vector routing protocols.

These updates pass routing tables from router to router to get the information out to all routers, but what does this really look like? If you examine the following illustration, you see three routers with their interfaces configured according to the diagram, as well as the routing table on each router. In this instance, no routing protocol is running.

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If you now enable a dynamic routing protocol and give it some time to propagate changes to all areas of the network or converge, the routing information will look more like the following figure. Note that the routers on each end of the network now know about the network segments at the other end of the network and have associated a hop count with the path to those networks.

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Setting up a routing protocol does not take much work; you need to give the routers only enough information to allow them to dynamically update their routing information between devices. Sharing of connection information or routing tables allows all routers in the routing group to know how to get to all the other network segments supported by the routing group.

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