A network administrator is troubleshooting a DMVPN setup between the hub and the spoke. Which action should the administrator take before troubleshooting the IPsec configuration?

DMVPN builds secure tunnels over GRE, and IPsec provides encryption and authentication over the GRE tunnel; therefore, the GRE tunnel must be up and functioning correctly for IPsec to be established and encrypt traffic over it. Verifying the GRE tunnel ensures basic, unencrypted overlay connectivity before troubleshooting IPsec.

ISAKMP (Internet Security Association and Key Management Protocol) is part of the IPsec negotiation process, and troubleshooting ISAKMP comes after ensuring the underlying GRE tunnel is functional.

NHRP (Next Hop Resolution Protocol) is crucial for DMVPN tunnel establishment, but the GRE tunnel must be up before NHRP can function correctly over it.

Crypto maps are part of the IPsec configuration; troubleshooting crypto maps would be a step in troubleshooting IPsec, which should occur after verifying the GRE tunnel.