Cisco Systems 78-11741-02 manual Controlling the Advertising of Routes in Routing Updates, IPC-372

Configuring IP Routing Protocol-Independent Features

Filtering Routing Information

Controlling the Advertising of Routes in Routing Updates

To prevent other routers from learning one or more routes, you can suppress routes from being advertised in routing updates. Suppressing routes in route updates prevents other routers from learning the interpretation of a particular device of one or more routes. You cannot specify an interface name in OSPF. When used for OSPF, this feature applies only to external routes.

To suppress routes from being advertised in routing updates, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# distribute-list	Permits or denies routes from being advertised in
{access-list-number access-list-name}out	routing updates depending upon the action listed in the
[interface-name routing-process as-number]	access list.

Controlling the Processing of Routing Updates

You might want to avoid processing certain routes listed in incoming updates. This feature does not apply to OSPF or IS-IS. To suppress routes in incoming updates, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# distribute-list	Suppresses routes listed in updates from being
{access-list-number access-list-name}in	processed.
[interface-type interface-number]

Filtering Sources of Routing Information

Filtering sources of routing information prioritizes routing information from different sources, because some pieces of routing information may be more accurate than others. An administrative distance is a rating of the trustworthiness of a routing information source, such as an individual router or a group of routers. In a large network, some routing protocols and some routers can be more reliable than others as sources of routing information. Also, when multiple routing processes are running in the same router for IP, it is possible for the same route to be advertised by more than one routing process. By specifying administrative distance values, you enable the router to intelligently discriminate between sources of routing information. The router will always pick the route whose routing protocol has the lowest administrative distance.

To filter sources of routing information, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# distance {ip-address{wildcard-	Filters on routing information sources.
mask}} [ip-standard-list][ip-extended]

There are no general guidelines for assigning administrative distances because each network has its own requirements. You must determine a reasonable matrix of administrative distances for the network as a whole. Table 9 shows the default administrative distance for various routing information sources.

Cisco IOS IP Configuration Guide

IPC-372