| 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.
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