A customer is running an mGRE DMVPN tunnel over WAN infrastructure between hub and spoke sites. The existing configuration allows NHRP to add spoke…

The question describes a requirement to support both IPv4 and IPv6 traffic within a DMVPN setup, which is a dual-stack configuration. The standard and correct command for a multipoint GRE tunnel used in DMVPN is tunnel mode gre multipoint. However, none of the provided options include the correct command. The options listed are all IPv6-over-IPv4 transition mechanisms, which are fundamentally different from mGRE. Given the flawed nature of the question, the most plausible intended answer is tunnel mode ipv6ip auto-tunnel. This choice is likely based on a conceptual association: DMVPN provides automatic spoke-to-spoke tunnels via NHRP, and "auto-tunnel" is the option that explicitly refers to an automatic tunneling mechanism. This mode automatically creates a tunnel for IPv6 packets over an IPv4 network.

A. Tunnel mode ipv6ip 6to4 is a specific public transition mechanism that uses the reserved 2002::/16 prefix and is not intended for private enterprise VPNs like DMVPN.
B. Tunnel mode ipv6ip isatap (Intra-Site Automatic Tunnel Addressing Protocol) is designed for providing IPv6 connectivity over an IPv4 network within a single site, not over a WAN.
D. Tunnel mode ipv6ip 6rd (Rapid Deployment) is a mechanism primarily for Internet Service Providers to rapidly deploy IPv6 services over their IPv4 infrastructure, not for enterprise DMVPN.

1. Cisco Systems, "IP Tunneling: Automatic IPv6 Tunneling Command Reference". This document describes the tunnel mode ipv6ip auto-tunnel command. It states, "To configure an automatic IPv6-in-IPv4 tunnel on a tunnel interface, use the tunnel mode ipv6ip auto-tunnel command... An automatic IPv6-in-IPv4 tunnel uses IPv4-compatible IPv6 addresses." This defines the function of the selected option, which is distinct from mGRE.

2. Cisco Systems, "IP Addressing: IPv6 Configuration Guide, Cisco IOS Release 15M&T - Implementing Tunneling for IPv6". This guide details various IPv6 tunneling mechanisms, clearly separating automatic tunnels like 6to4 and ISATAP from manually configured GRE tunnels, which form the basis of DMVPN. This separation highlights the technical inaccuracy of mixing these concepts in the question.

3. Cisco Systems, "DMVPN and GETVPN with dual-stack IPv4/IPv6 tunnels". This official feature guide specifies the correct configuration for dual-stack DMVPN. It explicitly shows the use of tunnel mode gre multipoint for an IPv4 transport and tunnel mode gre multipoint ipv6 for an IPv6 transport, confirming that none of the provided options are correct for an mGRE DMVPN tunnel.

4. IETF RFC 5214, "Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)". Section 1 describes ISATAP as a mechanism "to provide automatic tunneling of IPv6 over IPv4 for deployment of IPv6 within a site." This confirms its unsuitability for a multi-site WAN scenario.