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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.
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