Which common issue causes intermittent DMVPN tunnel flaps?

DMVPN relies on proper routing in the underlay to establish and maintain NHRP mappings and IPsec SAs; if there's an intermittent routing neighbor reachability issue (e.g., transient path failures), the NHRP resolution might fail, leading to tunnel flaps.

A suboptimal routing table might lead to inefficient traffic paths but typically would not cause intermittent tunnel flaps; it would more likely result in consistent routing problems.

Interface bandwidth congestion can cause packet loss and performance degradation, but it does not directly cause the tunnel to flap unless critical control plane traffic is consistently dropped, which is a less direct cause than a routing issue.

The GRE tunnel itself is not encrypted; IPsec encrypts the traffic over the GRE tunnel, and while IPsec failure prevents secure communication, it doesn't directly cause the GRE tunnel to intermittently flap.