NHRP - Holding Time

NHRP uses what is known as the Holding Time to determine for how long the next hop Non-Broadcast Multiple Access (NBMA) information specified in an NHRP update is considered valid. Cached information is discarded when the holding time expires.

The Holding Time is a value that is sent within the NHRP messages including Registration Requests. The RFC does not specify a default value, however, a typical default found in Cisco devices is 7200 seconds or two hours.

There is an additional value of time that can be configured on devices using NHRP, but this timer is not sent in any NHRP messages, nor is it negotiated between NHRP devices. This is the frequency with which registration request packets will be sent. By default, and according to the recommendation found in the RFC, this should be one third of the Holding Time.

For a default value of 7200 seconds the registration interval is recommended to be 2400 seconds or 40 minutes.

According Cisco documentation (linked below) the default holding time is indeed 7200 seconds or two hours, and registrations are sent every 2400 seconds (one third of the holding time). However, Cisco recommends changing the value of the holding timer to a value between 300 and 600. Some more modern Cisco devices will have a default of 600.

Cisco allows you to change the holding timer using the ip nhrp holdtime command, and it also allows you to change the reregistration from the default of 1/3 of that to whatever you like, using the ip nhrp registration timeout command.