150 Chapter 5: RIP, IGRP, and Static Route Concepts and Configuration

Distance Vector Concepts

Distance vector logic is pretty simple on the surface. However, the distance vector features that help prevent routing loops can actually be pretty difficult to grasp at first.

Distance vector protocols work by having each router advertise all the routes they know out all their interfaces. Other routers that share the same physical network receive the routing updates and learn the routes. The routers that share a common physical network are generally called neighbors. For instance, all routers attached to the same Ethernet are neighbors; the two routers on either end of a point-to-point serial link are also neighbors. If all routers advertise all their routes out all their interfaces, and all their neighbors receive the routing updates, eventually every router will know the routes to all the subnets in the network. It’s that simple!

The following list spells out the basic distance vector logic and introduces a few important concepts (which are explained over the next several pages):

■Routers add directly connected subnets to their routing tables, even without a routing protocol.

■Routers send routing updates out their interfaces to advertise the routes that this router already knows. These routes include directly connected routes, as well as routes learned from other routers.

■Routers listen for routing updates from their neighbors so that they can learn new routes.

■The routing information includes the subnet number and a metric. The metric defines how good the route is; lower metric routes are considered better routes.

■When possible, routers use broadcasts or multicasts to send routing updates. By using a broadcast or multicast packet, all neighbors on a LAN can receive the same routing information in a single update.

■If a router learns multiple routes to the same subnet, it chooses the best route based on the metric.

■Routers send periodic updates and expect to receive periodic updates from neighboring routers.

■Failure to receive updates from a neighbor in a timely manner results in the removal of the routes previously learned from that neighbor.

■A router assumes that, for a route advertised by Router X, the next-hop router in that route is Router X.

The following examples explain the concepts in the list in a little more depth. Figure 5-2 demonstrates how Router A’s directly connected subnets are advertised to Router B. In this case, Router A advertises two directly connected routes.