Identify two prerequisites for establishing a DMVPN mGRE/IPsec tunnel.

Defining an IKE policy with crypto isakmp policy is a fundamental prerequisite for IPsec VPNs, including DMVPN, as it establishes the security parameters for the IKE Phase 1 negotiation. Without it, the peers cannot agree on how to secure the key exchange. Enabling MPLS (mpls ip) is incorrect; MPLS is a separate forwarding technology and not a prerequisite for DMVPN tunnel establishment. While reachability between public IPs is necessary, a simple ping is not a formal prerequisite step in configuration. Wildcard preshared keys are a common configuration choice for DMVPN but not a strict prerequisite; specific keys can also be used. DMVPN is a Cisco proprietary solution, so it does not work on all OEM devices that support IKE.