Hello David
The NHRP purge request, as you correctly shared, is a message that is sent when a network entry is removed from the RIB. The message instructs the receiving device to remove a particular entry from the NHRP cache. What is the purpose of this? Of course, a spoke could send a packet once again to the hub, which would renew and update the NHRP entries. However, it is always preferable to keep the NHRP cache updated.
Over time, the NHRP cache may accumulate entries that are no longer valid. For example, a spoke might change its physical IP address, or a tunnel might become inactive. Keeping outdated entries can lead to inefficient routing and potential security issues.
The NHRP Purge Request is used to trigger the removal of such stale or invalid entries from the NHRP cache. It can be initiated by a network device (like a router) that detects a change in the network topology, or as a routine maintenance procedure.
Regular purging of the NHRP cache helps maintain optimal network performance in a DMVPN setup. It ensures that the path information is current, which is critical for the efficient and secure routing of traffic within the VPN network.
I hope this has been helpful!
Laz