7.3 Route Redistribution
7.3.6
Configuring one-way redistribution
Although the redistribution command is available for all IP routing protocols, it behaves differently depending on the actual IP routing protocols involved. The underlying principles, however, are the same, so the examples in this section can be used as a starting point for any redistribution scheme.

This section closely examines examples of one-way and two-way redistribution and then focuses on specific redistribution issues, including connected, static routes and the default-metric command.

In Figure , RTB injects routes learned via RIP into the EIGRP domain. However, the RIP routers do not learn about the EIGRP routes. This is one-way route distribution. In this example, the RIP routers can use a default route to handle any traffic bound for non-local destinations.

As the AS boundary router, RTB must run two routing processes: one for the RIP domain and one for the EIGRP AS, as shown in Figure .

The redistribute rip command enables route redistribution: RIP routes learned by RTB will be imported into the EIGRP process. The metric argument sets up the values used by EIGRP to translate the metric from RIP's hop count to EIGRP's composite metric. When used with IGRP/EIGRP, the metric keyword sets the bandwidth value (in kbps), the delay (in tens of microseconds), the reliability (out of 255), the load (out of 255), and, finally, the maximum transmission unit (MTU).

These five values constitute the seed metric in the example . The seed metric is the initial metric value of an imported route. After it is imported into the EIGRP AS, the RIP route becomes an EIGRP route with a composite metric derived from these seed values. So, using the above configuration, RIP routes with metrics of 2, 6, and 14 will all be redistributed with the same EIGRP metric value (2195456). However, as the imported route propagates to other EIGRP routers, its metric values increment according to EIGRP rules.

You can examine the routing tables of the EIGRP router, RTA; the boundary router, RTB; and the RIP router, RTC by clicking on the topology Figure . (The tables have been reformatted for the sake of clarity.)

RTA's routing table includes not only the EIGRP routes from AS 24, but also the redistributed routes from the RIP domain. The redistributed RIP routes that have been learned from RTB are denoted by "D EX" because EIGRP considers them external. As discussed in Chapter 6, EIGRP differentiates between internal routes (routes learned from within the AS) and external routes (imported from outside the AS). The Cisco IOS even assigns a different-and much less desirable-administrative distance to external EIGRP routes: 170.

RTB's table shows that RTB is running two routing protocols and has learned routes via RIP (denoted by R) and learned routes via EIGRP (denoted by D).

Notice that RTC does not have a default route and has not learned about any routes from the boundary router, RTB. That means that RTC can not route to 6 of the 12 networks shown in the outputs of Figure . You may decide that the best solution in this scenario is to use a default route that points to RTB. This can easily be accomplished statically, as shown:

RTC(config)# ip route 0.0.0.0 0.0.0.0 172.16.0.1

Because RTC is running RIP, it can dynamically propagate its 0.0.0.0/0 route to the other routers in the RIP domain. If you choose to implement this default route configuration, there is no need for the boundary router (RTB) to send updates into the RIP domain. Thus, you should configure RTB's RIP interface as passive, as shown:

RTB(config)# router rip
RTB(config-router)# passive-interface s0

A more complex topology may require that we employ two-way, or mutual, redistribution by importing the EIGRP routes into the RIP domain, as described in the next section.