Optimizing Routing

There are times when you need to go beyond just turning on a routing protocol in your network. You may need to use multiple protocols, control exactly which routes are advertised or redistributed, or which paths are chosen. Most networks use DHCP; your router may need to be a DHCP server, or relay DHCP broadcasts.

Using Multiple Routing Protocols
There are several reasons you may need to run multiple routing protocols in your network. Some include:

• Migrating from one routing protocol to another, where both protocols will run in the network temporarily
• Applications that run under certain routing protocols but not others
• Areas of the network under different administrative control (“layer 8” issues)
• A multi-vendor environment in which some parts of the network require a standards-based protocol

Configuring Route Redistribution
If routing information must be exchanged among the different protocols or routing domains, redistribution can be used. Only routes that are in the routing table and learned via the specified protocol are redistributed. Each protocol has some unique characteristics when redistributing, as shown in Table 5-1.


You can redistribute only between protocols that use the same protocol stack, such as IP protocols, which cannot advertise IPX routes. To configure redistribution, issue this command under the routing process that is to receive the new routes:

Router(config-router)#redistribute {route-source} [metric metric] [route-map tag]


Seed Metric
Redistribution involves configuring a routing protocol to advertise routes learned by another routing process. Normally, protocols base their metric on an interface value, such as bandwidth, but no interface for a redistributed route exists. Protocols use incompatible metrics, so the redistributed routes must be assigned a new metric compatible with the new protocol.

A route’s starting metric is called its seed metric. Set the seed metric for all redistributed routes with the default-metric [metric] command under the routing process. To set the metric for specific routes, either use the metric keyword when redistributing or use the route-map keyword to link a route map to the redistribution. After the seed metric is specified, it increments normally as the route is advertised through the network (except for certain OSPF routes).