GRE encapsulation format
As shown in Figure 93, a GRE-tunneled packet includes the following parts:
Payload packet—Original packet. The protocol type of the payload packet is called the passenger protocol. The passenger protocol can be any network layer protocol.
GRE header—Header that is added to the payload packet to change the payload packet to a GRE packet. A GRE header includes the number of encapsulations, version, passenger protocol type, checksum, and key. GRE is called the encapsulation protocol.
Delivery header—Header that is added to the GRE packet to deliver it to the tunnel end. The transport protocol (or delivery protocol) is the network layer protocol that transfers GRE packets.
Figure 93: GRE encapsulation format
