Because it supports
multiple routing protocols, EIGRP can be used to redistribute with
IP, IPX, and AppleTalk as well. Consider the following when
redistributing EIGRP with these protocols:
- In the IP
environment, IGRP and EIGRP have a similar metric structure and,
therefore, redistribution is straightforward. For migration
purposes, when IGRP and EIGRP are both running in the same AS
and are using the same AS numbers, redistribution is automatic.
When redistributing between different ASs, redistribution must
be configured for EIGRP, just as it is required for IGRP.
- By design, EIGRP
automatically redistributes route information with Novell RIP.
Beginning with Cisco IOS Release 11.1, EIGRP can redistribute
route information with NetWare Link Services Protocol (NLSP),
when configured.
- EIGRP for
AppleTalk understands Routing Table Maintenance Protocol (RTMP)
updates, and redistribution is enabled by default.
- All other IP
routing protocols, both internal and external, require that
redistribution be configured in order to communicate with EIGRP.
Using and Configuring
Redistribution
This section covers
how to configure redistribution, including modifying the
administrative distance and defining the seed metric. In addition,
two redistribution examples that include before and after show ip
route output are discussed.
Configuring route
redistribution can be very simple or very complex, depending on the
mix of protocols that you want to redistribute. The commands used to
enable redistribution and assigned metrics vary slightly, depending
on the protocols being redistributed. The following steps are
generic enough to apply to virtually all protocol combinations;
however, the commands used to implement the steps may vary. It is
highly recommended that you review the Cisco IOS documentation for
the configuration commands that apply to the specific protocols you
want to redistribute.
- Determine which
routing protocol is the core or backbone protocol. Usually it is
OSPF or EIGRP.
- Locate the ASBR
on which redistribution needs to be configured.
- Determine which
routing protocol is the edge or short-term (if you are migrating)
protocol.
- Access the
routing process into which you want routes redistributed.
Typically, you start with the backbone routing process. For
example, to access OSPF, do the following:
router(config)#router
ospf process-id
The routing
protocol on which you configure redistribution is the one into which
you want to redistribute. Another way to look at it is you are
preparing the receiving protocol, much like a house must be
prepared to receive guests.
The generic terms core
and edge are used to simplify the discussion about
redistribution. "Core" routing protocol refers to the
routing protocol used on the backbone. "Edge" routing
protocol refers to the other protocol that must perform
redistribution with the core routing protocol.
- Configure the
router to redistribute routing updates from the short-term
protocol into the backbone protocol. This command varies,
depending on the protocol. The following command is for OSPF:
router(config-router)#redistribute
protocol [process-id]
[metric metric-value]
[metric-type type-value] [subnets] 
Note: You
need to use the redistribute and
default-metric
commands to redistribute routes only between routing protocols
that do not automatically perform route redistribution.
- Define the seed
metric that the receiving router uses to calculate the value of
the route before redistributing the route.
When redistributing
IGRP or EIGRP, use the following:
router(config-router)#default-metric
bandwidth delay reliability loading mtu 
When
redistributing OSPF, RIP, Exterior Gateway Protocol (EGP),
and Border Gateway Protocol (BGP), use the following:
router(config-router)#default-metric
number 
- Exit the routing
process.
Note : To
avoid routing loops, it is highly recommended that you set the
default metric larger than the largest native metric, as discussed
earlier in this chapter.
- Enter
configuration mode for the other routing process, usually the
short-term process.
- Depending on
your network, this configuration will vary because you want to
employ some techniques to reduce routing loops. For example, you
may do any of the following:
- Redistribute a
default route about the backbone AS into the border AS.
- Redistribute
multiple static routes about the backbone AS into the other AS.
- Redistribute all
routes from the backbone AS into the border AS, and then assign
a distribution filter.
- Redistribute all
routes from the backbone AS into the border AS, and then modify
the distance associated with the received routes so they are not
selected when multiple routes exist for the same destination. In
some cases, the route learned by the native protocol is better,
but may have a less believable administrative distance.
The following
sections illustrate two redistribution configuration examples. It is
important to note, however, that every network is different and may
require a different combination of commands to make redistribution
operate properly.
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