Command:

set ip next-hop verify-availability


Mode:

Router(config-route-map)#


Syntax:

set ip next-hop verify-availability

 

Syntax Description:

This command has no arguments or keywords.

 

Command Description:

To configure policy routing to verify if the next hops of a route map are Cisco Discovery Protocol (CDP) neighbors before policy routing to those next hops, use the set ip next-hop verify-availability command in route-map configuration mode.

One example of when you might configure this command is if you have some traffic traveling via a satellite to a next hop. It might be prudent to verify that the next hop is reachable before trying to policy route to it.

This command has the following restrictions:

It causes some performance degradation.
CDP must be configured on the interface.
The next hop must be a Cisco device with CDP enabled.
It is supported in process switching and Cisco express forwarding (CEF) policy routing, but not available in dCEF, due to the dependency of the CDP neighbor database.

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, use this command to configure the router to first verify that the next hops of the route map are the CDP neighbors of the router before routing to those next hops.

This command 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.

If this command is set and the next hop is not a CDP neighbor, the router looks to the subsequent next hop, if there is one. If there is none, the packets simply are not policy routed.

If this command is not set, the packets are either successfully policy routed or remain forever unrouted.

If you want to selectively verify availability of only some next hops, you can configure different route map entries (under the same route map name) with different criteria (using access list matching or packet size matching), and use the set ip next-hop verify-availability command selectively.


Example:

The following example configures CEF, NetFlow, and NetFlow with flow acceleration. It also configures policy routing to verify that next hop 10.0.0.8 of the route map named test is a CDP neighbor before the router tries to policy route to it.

If the first packet is being policy routed via route map test sequence 10, the subsequent packets of the same flow always take the same route map test sequence 10, not route map test sequence 20, because they all match or pass the access list 1 check. Therefore, policy routing can be flow-accelerated by bypassing the access list check.

Router(config)# ip cef
Router(config)# ip flow-cache feature-accelerate
Router(config)# interface ethernet0/0/1
Router(config)# ip route-cache flow
Router(config)# ip policy route-map test
Router(config)# route-map test permit 10
Router(config-route-map)# match ip address 1
Router(config-route-map)# set ip precedence priority
Router(config-route-map)# set ip next-hop 10.0.0.8
Router(config-route-map)# set ip next-hop verify-availability 
Router(config)# route-map test permit 20
Router(config-route-map)# match ip address 101
Router(config-route-map)# set interface Ethernet0/0/3
Router(config-route-map)# set ip tos max-throughput


Misconceptions:

None


Related Commands:

None

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