Route Redistribution

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While you are not really going to redistribute routes as a part of CCNA, it is important to know what it is. Simply put, route redistribution is the process of distributing routes learned from one source to another.  This is useful when networks are expanding, or are merging, or in a phase of transition.

For example, assume that RIP is being used in a growing network. Beyond a hop count of 15, it will become impossible to use RIP. In this situation, you will need to switch to another routing protocol. While switching, two protocols would need to co-exist in the network while maintaining complete reachability. Redistribution of routes from RIP to the new protocol and vice versa can achieve this.

Another example where you would need to use redistribution is when a company acquires another and their networks need to merge. If both the networks were using different routing protocols, redistribution between these protocols can provide full connectivity with the least amount of effort.

Figure 4-5 Route Redistribution

A Few important points that you should remember about route redistribution are:

1. The routing protocol receiving the redistributed routes will mark them as external. External routes are less preferred that Internal routes
2. Routes can only be redistributed at routers that run both the routing protocols. For example, Figure 4-3 shows RouterB running EIGRP and OSPF both. I the network shown, routes can be redistributed on RouterB only.
3. It is possible to redistribute between two different processes or AS of the same protocol. For example, if you have two EIGRP AS running on a Router, you can redistribute between them.
4. Static and Connected routes can also be redistributed
5. Only routes present in routing tables can be redistributed. For example, if a static route points to an unknown next-hop, it will not be present in the routing table and cannot be redistributed.
6. When redistributing routes, you have to ensure metric compatibility. For example, EIGRP metrics can be large numbers while any metric above 15 is considered invalid in RIP. In such cases, you have to tell the receiving routing protocol how to translate the metrics.

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