7.5 Using Multiple Routing Protocols
7.5.9 Redistribution example using distance
This example uses RIP and IGRP to illustrate how a router can make a poor path selection because of the administrative distance values given to RIP and IGRP in a redundant network. The example also illustrates one possible way of correcting the problem.

Figure illustrates the network prior to using multiple routing protocols. The R200 and Cen routers are the primary focus of this example, as are networks 172.16.6.0, 172.16.9.0, and 172.16.10.0. The configuration output and routing tables appear on the following pages.

There are a number of ways to correct path selection problems in a redistribution environment. The purpose of this example is to show how a problem can occur, where it appears, and one possible way of resolving it.

Note: The example solution that follows uses RIP and IGRP for simplicity. Because these and other protocol combinations can have the same problem occur, depending on the network topology, Cisco highly recommends that you study your network topology prior to implementing redistribution, and to monitor it after it is enabled.

The output in the figure to the left is the complete IP route table for the Cen router. (see Cen#show ip route command output).

Note the administrative distance and the composite metrics for each learned link. Recall that administrative distance refers to how believable the routing protocol is, and the composite metric is the value assigned to the link. Now consider that you want to split the network into two ASs --- IGRP and RIP. Note that IGRP is more believable than RIP because it has an administrative distance of 100 and RIP has an administrative distance of 120.

Figure shows the network with RIP and IGRP ASs identified.

The configurations for two of the routers are as shown in Figure command outputs. (see   R200#show running-config and RouterCen#show running-config command outputs). 

The passive interface command is used to prevent RIP routes from being forwarded needlessly on the links when the remote router cannot understand or is not using RIP, and similarly for IGRP.

Now, see what the route tables of the Cen router look like. The output shows the output from running the show ip route command on the Cen router. (see Cen#show ip route command output).

The Cen route table lists the routes that are relevant to the discussion in this section. Notice that the Cen router learned RIP and IGRP routes.

If you look at the network topology, there are several destinations whose paths are better when learned by RIP because they are in the RIP domain. These destinations are 172.16.9.0, 172.16.10.0, and 172.16.6.0. Regardless of the router, you want the RIP versions of these routes to be learned.

The route table lists the routes that are relevant to the discussion in this section. The table shows the routes learned by R200. Notice that all the routes are learned from IGRP, even though R200 is also connected to a RIP network. Notice too that when you trace some of the routes, such as to network 172.16.9.0, the long way via router Cen rather than via router R300 appears in the following route table (see R200#show ip route  (1) command output).

Router R200 selected the poor paths because IGRP has a better administrative distance than RIP. To make sure that R200 selects the RIP routes, you can change the administrative distance, as shown in the Figure R200#show running-config.

On router R200, for example, it is configured to assign an administrative distance of 130 to IGRP routes to networks 172.16.9.0, 172.16.10.0, and 172.16.6.0. In this way, when the router learns about these networks from RIP, the RIP-learned routes will be selected and put in the routing table. Note that the distance command is for IGRP-learned routes because it is part of the IGRP routing process configuration.

Now consider the show ip route output from router R200. (see R200#show ip route (2) command output).

Router R200 now has retained the better route to some of the networks by learning them from RIP.

With this type of configuration, however, note the loss of routing information. That is, given the actual bandwidths involved, the IGRP path would have been better for the 172.16.10.0 network. But to select that route, you would need to retain the suboptimal path for 172.16.9.0.

Note: Important concepts of this example include the following:

  • Know your network traffic patterns before enabling redistribution.
  • Verify redistribution operation after configuration
  • If suboptimal paths exist, determine the solution based on user requirements.

This example illustrates that you should not only know your network prior to implementing redistribution, but you should also view which routes the routers are selecting after redistribution is enabled. You should pay particular attention to routers that can select from a number of possible redundant paths to a network because they are more likely to select suboptimal paths.