The use of a routing protocol to advertise routes that are learned by some other means, such as by another routing protocol, static routes, or directly connected routes, is called redistribution. While running a single routing protocol throughout your entire IP internetwork is desirable, multi-protocol routing is common for a number of reasons, such as company mergers, multiple departments managed by multiple network administrators, and multi-vendor environments. Running different routing protocols is often part of a network design. In any case, having a multiple protocol environment makes redistribution a necessity.
Example of Route Redistribution
Differences in routing protocol characteristics, such as metrics, administrative distance, classful and classless capabilities can effect redistribution. Consideration must be given to these differences for redistribution to succeed.
Redistribution is always performed outbound; the router doing redistribution does not change its routing table.The boundary router’s neighbors see the redistributed routes as external routes.Routes must be in the routing table for them to be redistributed.
The key issues that arise when using redistribution:
- Routing feedback (loops)
- If more than one boundary router is performing route redistribution, then the routers might send routing information received from one autonomous system back into that same autonomous system.
- Incompatible routing information
- Each routing protocol uses different metrics to determine the best path therefore path selection using the redistributed route information might not be optimal.
- Inconsistent convergence times
- Different routing protocols converge at different rates.
Good planning should solve the majority of issues but additional configuration might be required.
- Some issues might be solved by changing the administrative distance, manipulating the metrics, and filtering using route maps, distribute lists, and prefix lists.
Selecting the Best Route
Routers use the following two parameters to select the best path:
Used to rate a routing protocol’s believability (also called its trustworthiness).This criterion is the first thing a router uses to determine which routing protocol to believe if more than one protocol provides route information for the same destination.
The routing metric is a value representing the path between the local router and the destination network, according to the routing protocol being used.The metric is used to determine the routing protocol’s “best” path to the destination.