To resolve the adjacency, CEF marks the tunnel destination as “glean” – meaning it will request NHRP data when traffic needs to be forwarded over that interface.
When CEF is enabled, as long as traffic is sent to a node (which means a specific CEF entry is used), that entry doesn’t timeout. When the timer is less than 120 seconds, the CEF entry is marked as stale. If a new packet is switched over the stale entry, a new NHRP request is sent and the entry is refreshed. If no packet is switchd until the timer expires, the CEF entry times out becomes invalid.
When CEF is not used, the entries expire based on the “used” flag associated with the map entries. When there’s no more traffic using that entry, it will timeout.