Depending on the RIP network topology and the links between nodes in the network, you might want to control traffic flow through the network to maximize flow across higher-bandwidth links. Figure 70 shows a network with alternate routes between Routers A and D.
Figure 70: Controlling Traffic in a RIP Network with the Incoming Metric
In this example, routes to Router D are received by Router A across both of its RIP-enabled interfaces. Because the route through Router B and the route through Router C have the same number of hops, both routes are imported into the forwarding table. However, because the T3 link from Router B to Router D has a higher bandwidth than the T1 link from Router C to Router D, you want traffic to flow from A through B to D.
To force this flow, you can modify the route metrics as they are imported into Router A's routing table. By setting the incoming metric on the interface from Router A to Router C, you modify the metric on all routes received through that interface. Setting the incoming route metric on Router A changes only the routes in Router A's routing table, and affects only how Router A sends traffic to Router D. Router D's route selection is based on its own routing table, which, by default, includes no adjusted metric values.
In the example, Router C receives a route advertisement from Router D and readvertises the route to Router A. When Router A receives the route, it applies the incoming metric on the interface. Instead of incrementing the metric by 1 (the default), Router A increments it by 3 (the configured incoming metric), giving the route from Router A to Router D through Router C a total path metric of 4. Because the route through Router B has a metric of 2, it becomes the preferred route for all traffic from Router A to Router D.
To modify the incoming metric on all routes learned on the link between Router A and Router C and force traffic through Router B:
Table 121: Modifying the Incoming Metric