A FlexVPN (IKEv2) site-to-site connection repeatedly fails during IKE phase 1 when the peers are configured to use certificates. The local router's crypto logs show identity verification failures. What single configuration issue is most likely causing the failure?

The most likely cause for an IKEv2 Phase 1 failure with identity verification errors when using certificates is a mismatch between the peer's presented certificate identity and the identity expected by the local device. In IKEv2, peer identities are crucial for authentication. If the local router is configured to expect a specific identity (e.g., a hostname or distinguished name) for the remote peer, but the remote peer's certificate presents a different identity, the authentication will fail. Incorrect options: The transform-set for IPsec child SAs mismatching the IKE proposal would cause a Phase 2 failure, not a Phase 1 identity verification failure. NAT traversal being enabled on one side but disabled on the other might prevent the connection from establishing, but it wouldn't specifically manifest as an "identity verification failure" in the crypto logs. An interface with no IP address configured cannot function as an IKEv2 listener, preventing any negotiation from starting, but the logs would indicate a lack of reachability or listener, not an identity verification error.