GRE encapsulation format

A GRE-tunneled packet comprises the following parts:

• Payload packet—The packet to be encapsulated and transmitted. The protocol type of the payload is called the "passenger protocol."

• GRE header—After the system receives a payload packet, it adds a GRE header to the payload packet, so that the payload packet can be transferred as a GRE packet. The GRE protocol, which encapsulates the payload packet, is called the "encapsulation protocol."

• Delivery header—The protocol used to transfer the GRE packet over the network is called the "delivery protocol" or "transport protocol." The system adds a transport protocol header to the GRE packet to deliver it to the tunnel end.

For example, to transfer an IPv6 packet over an IPv4 network through a GRE tunnel, the system encapsulates the IPv6 packet in the format shown in Figure 95. The passenger protocol is IPv6, the encapsulation protocol is GRE, and the transport protocol is IPv4.

Depending on the transport protocol, GRE tunnels have the following tunnel modes:

• GRE over IPv4—The transport protocol is IPv4, and the passenger protocol is any network layer protocol.

• GRE over IPv6—The transport protocol is IPv6, and the passenger protocol is any network layer protocol.