Managing Redistribution

Ideally, all routers in your organization should run the same routing protocol. In reality, however, doing so can be difficult, especially when you are faced with any of the following situations:

• Various groups are managing their own routers and must use different routing protocols for one reason or another. Yet everyone must be internet worked together.

• You are migrating a large network of routers to a new routing protocol but you don't want to convert all routers at once. Instead, you want to migrate sections of the network in phases. During the migration, routers using the new protocol must still interoperate with routers using the old protocol.

• You have just acquired or merged with another organization and it doesn't use the same routing protocol. You need to establish connectivity between the organizations now. Later on, you might consider migrating the unified network to a single routing protocol.

• You run multiple autonomous systems or processes of the same routing protocol for administrative purposes and you need to share routing information across these routing domain boundaries.

• You need to exchange routing information with your Internet Service Provider, who runs a different routing protocol than you do.

Routing protocol redistribution resolves these issues. It is a translation service that allows different routing protocols to interoperate. Consider, for example, an organization's network depicted in Figure 3-3.

As shown in Figure 3-3, this organization has deployed EIGRP (OSPF works equally well) on a core group of routers to take advantage of such varied features as VLSM, fast convergence, better scalability, and route summarization. Off to the lower left is a legacy RIP network—these might be routers that can run only RIP or they might be routers that haven't yet migrated from RIP to EIGRP. To the lower right is a newly acquired company that needs connectivity to the rest of the organization. It also uses RIP.

Figure 3-3 A Scenario for Routing Protocol Redistribution

Legacy RIP network Acquired company

(also RIP)

Figure 3-3 A Scenario for Routing Protocol Redistribution

Legacy RIP network Acquired company

(also RIP)

Redistribution is a service that allows the sharing of routes across the various RIP and EIGRP routing domains. It transfers routes from the RIP domain to the EIGRP domain, and vice versa, so that every route is propagated and full connectivity is achieved (assuming there are no route filters in place). For illustration purposes, consider the propagation of the routes in the legacy RIP network (refer to Figure 3-3):

1 Routes from the legacy RIP network ( and are redistributed into EIGRP. The routes are converted from the RIP format to an EIGRP format and injected into the core.

2 The routes propagate through the core as EIGRP routes until they reach the boundary between the core and the acquisition network (right side of Figure 3-3).

3 Routes and are redistributed again, but this time from EIGRP into RIP. As the routes are injected into the acquisition network, they are converted from EIGRP to RIP.

4 Routes and are accepted and propagated through the acquisition network as RIP routes.

5 The result; Routers in the core and in the acquisition network now know how to reach and because of redistribution. A similar exercise can be done for tracing the redistribution of routes sourced by the acquisition network.

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