Cisco Systems 78-11741-02 manual Enabling Policy Routing PBR, IPC-373

Configuring IP Routing Protocol-Independent Features

Enabling Policy Routing (PBR)

For example, consider a router using IGRP and RIP. Suppose you trust the IGRP-derived routing information more than the RIP-derived routing information. In this example, because the default IGRP administrative distance is lower than the default RIP administrative distance, the router uses the IGRP-derived information and ignores the RIP-derived information. However, if you lose the source of the IGRP-derived information (because of a power shutdown in another building, for example), the router uses the RIP-derived information until the IGRP-derived information reappears.

For an example of filtering on sources of routing information, see the section “Administrative Distance Examples” later in this chapter.

Note You also can use administrative distance to rate the routing information from routers running the same routing protocol. This application is generally discouraged if you are unfamiliar with this particular use of administrative distance, because it can result in inconsistent routing information, including forwarding loops.

Note The weight of a route can no longer be set with the distance command. To set the weight for a route, use a route-map.

Enabling Policy Routing (PBR)

Policy routing (or “policy-based routing” [PBR]) is a more flexible mechanism for routing packets than destination routing. It is a process whereby the router puts packets through a route map before routing them. The route map determines which packets are routed to which router next. You might enable policy routing if you want certain packets to be routed some way other than the obvious shortest path. Possible applications for policy routing are to provide equal access, protocol-sensitive routing, source-sensitive routing, routing based on interactive versus batch traffic, and routing based on dedicated links.

To enable policy routing, you must identify which route map to use for policy routing and create the route map. The route map itself specifies the match criteria and the resulting action if all of the match clauses are met. These steps are described in the following task tables.

To enable policy routing on an interface, indicate which route map the router should use by using the following command in interface configuration mode. A packet arriving on the specified interface will be subject to policy routing except when its destination IP address is the same as the IP address of the router’s interface. This command disables fast switching of all packets arriving on this interface.

Command	Purpose
Router(config-if)#ip policy route-map map-tag	Identifies the route map to use for policy routing.

To define the route map to be used for policy routing, use the following command in global configuration mode:

Command	Purpose
Router(config)# route-map map-tag[permit deny]	Defines a route map to control where packets are output.
[sequence-number]

Cisco IOS IP Configuration Guide

IPC-373