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Configuring IP Routing Protocol-Independent Features
Filtering Routing Information
IPC-370
Cisco IOS IP Configuration Guide
• 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:
See the “Passive Interface Examples” section at the end of this chapter for examples of configuring
passive interfaces.
Command Purpose
Router(config-router)# passive-interface
interface-type interface-number
Suppresses the sending of routing updates through the
specified interface.