Content

| 8.3 |  | Route Redistribution |
| 8.3.1 |  | Redistribution overview |

To support multiple routing protocols within the same internetwork efficiently, routing information must be shared among the different routing protocols. For example, routes learned from a RIP process may need to be imported into an IGRP process. This process of exchanging routing information between routing protocols is called route redistribution. Such redistribution can be one-way or two-way. One-way routes are where one protocol receives the routes from another. Two-way routes are where both protocols receive routes from each other. Routers that perform redistribution are called boundary routers because they border two or more autonomous systems or routing domains. This section examines route redistribution in detail, including the use of administrative distance, guidelines for redistribution implementation, and issues with redistribution configuration.

Using multiple routing protocols typically results in increased administrative complexity and overhead. Why would this be done in the first place? Actually, there are several scenarios in which using multiple routing protocols solves more problems than it creates, especially in medium and large sized networks.

Consider a large, mixed vendor routing environment in which Cisco routers work alongside other routers. An administrator may create an all Cisco domain, where the advantages of proprietary protocols such as IGRP and EIGRP can be retained. Meanwhile, other areas of the network run a nonproprietary protocol, such as OSPF or RIP.

Multiple routing protocols may also be effectively deployed to support legacy UNIX systems that support RIP only. These systems may represent a significant financial investment and may not be readily upgradeable. An administrator may elect to run RIP on subnets populated by the UNIX systems but might use a more scalable protocol elsewhere. Also, running multiple routing protocols can be seen as a temporary fix during a prolonged upgrade from older protocols and hardware to newer, more scalable solutions.

On some occasions redistribution is implemented even when running compatible routing platforms. For example, an organization with all Cisco routers running EIGRP. If the organization is exchanging routing information with a domain outside its administrative control, it may choose to configure route redistribution as a means of logically separating the different routing processes, which may have different policies.

Cisco routers support up to 30 dynamic routing processes. This means that a router can run RIP, OSPF, IGRP, IS-IS, EIGRP, IPX RIP, RTMP, AppleTalk, and other protocols simultaneously. Most of these routing protocols enable the configuration of multiple processes of the same routing algorithm. RIP is a notable exception. For example, multiple IGRP processes can be defined by using different AS numbers, or different OSPF processes by using different process ID numbers.

Notice that in the configuration shown in Figure , that the OSPF processes for RTA, 24 and 46, will not share routing information unless route redistribution is configured. Each routing process places substantial demands on the router memory and CPU resources. Because of this, only boundary routers should run more than one routing process for the same routed protocol, and only when absolutely necessary.