Using Route Redistribution

The following are possible reasons why you might need multiple routing protocols running at the same time within your network:

■ You are migrating from an older IGP to a new IGP. Multiple redistribution boundaries might exist until the new protocol has displaced the old protocol completely. Running multiple routing protocols during a migration is effectively the same as a network that has multiple routing protocols running as part of its design.

■ You want to use another protocol but have to keep the old routing protocol because of the host system's needs. For example, UNIX host-based routers might run only RIP.

■ Some departments might not want to upgrade their routers to support a new routing protocol.

■ If you have a mixed-vendor environment, you can use the Cisco-proprietary EIGRP routing protocol in the Cisco portion of the network and then use a common standards-based routing protocol, such as OSPF, to communicate with non-Cisco devices.

When any of these situations arises, Cisco routers allow internetworks using different routing protocols (referred to as routing domains or autonomous systems) to exchange routing information through a feature called route redistribution. This allows, for example, hosts in one part of the network to reach hosts in another part that is running a different routing protocol.

KEY POINT

In some cases, the same protocol may be used in multiple different domains or autonomous systems within a network. Multiple instances of the protocol are treated no differently than if they were distinct protocols; redistribution is required to exchange routes between them. Accordingly, redistribution of routes is required when one or both of the following occur:

■ Multiple routing protocols are used in the network—for example, RIPv2, EIGRP, and OSPF.

■ Multiple routing domains are used in the network—for example, two EIGRP routing processes.

Redistribution occurs on the boundaries between routing protocols and between domains. As shown in Figure 7-15, redistribution occurs on a router with interfaces that participate in multiple routing protocols or routing domains.

Figure 7-15 Redistribution Occurs on the Boundaries Between Protocols or Domains

Figure 7-15 Redistribution Occurs on the Boundaries Between Protocols or Domains

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