Cisco Systems 78-11741-02 manual Filtering Routing Information, Specified interface, IPC-370

Configuring IP Routing Protocol-Independent Features

Filtering Routing Information

•IGRP can automatically redistribute static routes and information from other IGRP-routed autonomous systems. IGRP assigns static routes a metric that identifies them as directly connected. IGRP does not change the metrics of routes derived from IGRP updates from other autonomous systems.

•Note that any protocol can redistribute other routing protocols if a default metric is in effect.

Filtering Routing Information

To filter routing protocol information performing the tasks in the following sections. The tasks in the first section are required; the tasks in the remaining sections are optional:

•Preventing Routing Updates Through an Interface (Required)

•Controlling the Advertising of Routes in Routing Updates (Optional)

•Controlling the Processing of Routing Updates (Optional)

•Filtering Sources of Routing Information (Optional)

Note When routes are redistributed between OSPF processes, no OSPF metrics are preserved.

Preventing Routing Updates Through an Interface

To prevent other routers on a local network from learning about routes dynamically, you can keep routing update messages from being sent through a router interface. Keeping routing update messages from being sent through a router interface prevents other systems on the interface from learning about routes dynamically. This feature applies to all IP-based routing protocols except BGP.

OSPF and IS-IS behave somewhat differently. In OSPF, the interface address you specify as passive appears as a stub network in the OSPF domain. OSPF routing information is neither sent nor received through the specified router interface. In IS-IS, the specified IP addresses are advertised without actually running IS-IS on those interfaces.

To prevent routing updates through a specified interface, use the following command in router configuration mode:

Command	Purpose
Router(config-router)# passive-interface	Suppresses the sending of routing updates through the
interface-type interface-number	specified interface.

See the “Passive Interface Examples” section at the end of this chapter for examples of configuring passive interfaces.

Cisco IOS IP Configuration Guide

IPC-370