DMVPN - NHRP purge request

In a DMVPN topology, NHRP is used to resolve the next hop IP address to allow routing to operate correctly. In such a topology, the NHRP purge request is a mechanism used to remove outdated or invalid entries from the NHRP cache. Here's a detailed look at how it works:

  1. Purpose of NHRP in DMVPN: NHRP is used in DMVPN networks to facilitate the creation of direct tunnels between spoke sites in a hub-and-spoke VPN topology. It allows spokes to dynamically learn the physical IPv4 or IPv6 addresses of other spokes, so they can establish direct, secure tunnels without routing traffic through the hub.
  2. NHRP Cache: In the process of resolving network addresses, NHRP maintains a cache that maps the virtual addresses (used within the VPN) to the physical addresses (used on the underlying network) of the network nodes. This cache is crucial for efficient routing within the DMVPN.
  3. Need for Purging: Over time, the NHRP cache may accumulate entries that are no longer valid.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 Keeping outdated entries can lead to inefficient routing and potential security issues.
  4. NHRP Purge Request: The NHRP Purge Request is a message sent within the DMVPN network 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.
  5. Process: When an NHRP Purge Request is received, the receiving device checks its NHRP cache. Any entries that match the criteria specified in the purge request are removed. This ensures that the NHRP cache remains up-to-date and reflects the current network topology.
  6. Benefits: 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.

The NHRP Purge Request in a DMVPN context is essentially a maintenance mechanism that helps ensure the NHRP cache is accurate and up-to-date, thereby supporting efficient and secure network operations.