Cisco Systems 78-11741-02 manual Preverifying Next-Hop Availability, Number Name, IPC-375

Configuring IP Routing Protocol-Independent Features

Enabling Policy Routing (PBR)

Table 10	IP Precedence Values
Number		Name
0		routine
1		priority
2		immediate
3		flash
4		flash-override
5		critical
6		internet
7		network

The set commands can be used with each other. They are evaluated in the order shown in the previous task table. A usable next hop implies an interface. Once the local router finds a next hop and a usable interface, it routes the packet.

Preverifying Next-Hop Availability

If the router is policy routing packets to the next hop and the next hop happens to be down, the router will try unsuccessfully to use Address Resolution Protocol (ARP) for the next hop (which is down). This behavior will continue forever.

To prevent this situation, you can configure the router to first verify that the next hops of the route map are CDP neighbors of the router before routing to that next hop.

This task is optional because some media or encapsulations do not support CDP, or it may not be a Cisco device that is sending the router traffic

To configure the route-map policy to verify that the next hop is available before the router attempts to route traffic to it, use the following command in route-map configuration mode:

Command	Purpose
Router(config-route-map)# set ip next-hop	Causes the router to confirm that the next hop, specified in the route
verify-availability	map configuration, are active and available.
	• This command relies on CDP to determine if the next hop is an
	active CDP neighbor.
	• If this command is not used, and the next hop is not available,
	the traffic will remain forever unrouted.
	• If this command is used, and the next hop is not a CDP
	neighbor, the router looks to the subsequent next hop, if there
	is one. If a subsequent next-hop is not defined, the packets
	simply are not policy routed

The set ip next-hop verify-availability has the following restrictions:

•It can cause some performance degradation due to CDP database lookup overhead per packet.

•CDP must be enabled on the interface.

Cisco IOS IP Configuration Guide

IPC-375