7.5 Using Multiple Routing Protocols
7.5.2 Redistribution implementation considerations
Redistribution, although powerful, increases the complexity and potential for routing confusion, so it should be used only when absolutely necessary. The key issues that arise when using redistribution follow:
  • Routing feedback (loops) --- Depending on how you employ redistribution, routers can send routing information received from one AS back into the AS, as shown in the main figure. The feedback is similar to the split-horizon problem that occurs in distance vector technologies.

In the main figure, information about network 172.16 crosses the AS boundary router (ASBR) A to the EIGRP net. Because this internetwork has a loop, the same information propagates back to the RIP network through ASBR B.

  • Incompatible routing information --- Because each routing protocol uses different metrics to determine the best path (for example, RIP uses hops and OSPF uses cost), path selection using the redistributed route information may not be optimal. Because the metric information about a route cannot be translated exactly into a different protocol, the path a router chooses may not be the best.
  • Inconsistent convergence time --- Different routing protocols converge at different rates. For example, RIP converges slower than EIGRP, so if a link goes down, the EIGRP network will learn about it before the RIP network.

To understand why some of these problems occur, you must understand how the Cisco routers convert the metrics used when importing routes from one AS into another; this conversion is discussed later.

NoteRedistribution is the capability for boundary routers connecting different ASs to exchange and advertise routing information received from one AS to the other AS.

The next sections discuss how a router can select a path when it learns routes to a given destination from different routing protocols. Understanding how path selection occurs in this environment is important to prevent and troubleshoot routing feedback and poor path selection.