7.3 Route Redistribution
7.3.5 Redistribution guidelines
Route redistribution can be tricky business, with several pitfalls:
  • Routing loops - Depending on how you use redistribution, routers can send routing information received from one AS back into the AS. The feedback is similar to the split-horizon problem that occurs in distance-vector technologies.
  • Incompatible routing information - Each routing protocol uses different metrics. Because these metrics cannot be translated exactly into a different protocol, path selection using the redistributed route information may not be optimal.
  • 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.

These potential trouble spots can be avoided with careful planning and implementation. Be sure to follow these important guidelines when configuring route redistribution:

  • Be familiar with your network. There are many ways to implement redistribution, so knowing your network will enable you to make the best decision.
  • Do not overlap routing protocols. Do not run two different protocols in the same internetwork. Instead, have distinct boundaries between networks that use different routing protocols.
  • Use one-way redistribution with multiple boundary routers. If more than one router serves as a redistribution point, use one-way redistribution to avoid routing loops and convergence problems. Consider using default routes in the domains that do not import external routes.
  • Use two-way redistribution with a single boundary router. Two-way redistribution works smoothly when redistribution is configured on a single boundary router in the internetwork. If you have multiple redistribution points, do not use two-way redistribution unless you enable a mechanism to reduce the chances of routing loops. A combination of default routes, route filters, and distance modifications can be used to combat routing loops.