7.5 Using Multiple Routing Protocols
7.5.5 Redistribution and EIGRP
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.

  1. Determine which routing protocol is the core or backbone protocol. Usually it is OSPF or EIGRP.
  2. Locate the ASBR on which redistribution needs to be configured.
  3. Determine which routing protocol is the edge or short-term (if you are migrating) protocol.
  4. 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.

  1. 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.

  1. 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

  1. 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.

  1. Enter configuration mode for the other routing process, usually the short-term process.
  2. 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.