IPv6 payload packets can be delivered over an IPv4 GRE tunnel. In this scenario the two endpoints of the GRE tunnel have IPv4 addresses and the VPRN or IES SAP interface to the tunnel is an IPv6 only or dual-stack IPv4/IPv6 interface. IPv6 over IPv4 GRE tunneling allows IPv6 islands to be connected over an IPv4 only transport infrastructure.
To configure a tunnel to carry IPv6 payload, the tunnel must be configured with at least one dest-ip that contains an IPv6 address (global unicast and/or link local). A tunnel can have up to 16 dest-ip addresses (IPv4 and IPv6 together). For a tunnel to come operationally up all the dest-ip addresses must be part of locally configured subnets (associated with the private tunnel interface).
To forward IPv6 traffic through a tunnel supporting IPv6 payload, a dynamic routing protocol (such as BGP or OSPFv3) can be configured to run inside the tunnel (by associating the protocol with the private tunnel interface) or else an IPv6 static route next-hop equal to a dest-ip of the tunnel can be used.
IPv6 payload packets larger than 1280 bytes (the minimum IPv6 MTU) and also larger than the configured ip-mtu value of the tunnel are always discarded. If the icmp6-generation and packet-too-big commands are configured under the tunnel, then ICMPv6 Packet-Too-Big messages are generated and sent back to the originating host when discards occur because of the private side IP MTU being exceeded.