Content

| 8.3 |  | Route Redistribution |
| 8.3.4 |  | Redistribution guidelines |

Route redistribution can be complicated and have several disadvantages as shown in the following:

- Routing loops – Depending on how redistribution is used, 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. Use the following important guidelines when configuring route redistribution:

- Be familiar with the network – There are many ways to implement redistribution and being familiar with the network helps to make the best decisions.
- 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 there are multiple redistribution points, do not use two-way redistribution unless a mechanism to reduce the chances of routing loops is enabled. A combination of default routes, route filters, and distance modifications can be used to combat routing loops.

| Lab Activity e-Lab Activity: Modify Administrative Distance by Using the Distance Command This lab is to configure the RouterA to apply a custom administrative distance to all OSPF routes.