3Com Switch 8800 Advanced Software V5 Configuration Guide

406 CHAPTER 34: GRE CONFIGURATION
3 If the packet must be tunneled to reach its destination, Router A sends it to the
tunnel interface.
4 Upon receipt of the packet, the tunnel interface encapsulates it in a GRE packet
and submits to the IP module.
5 The IP module encapsulates the packet in an IP packet, and then forwards the IP
packet out through the corresponding network interface based on its destination
address and the routing table.
Format of an encapsulated packet
Figure 123 shows the format of an encapsulated packet.
Figure 123 Format of an encapsulated packet
As an example, Figure 124 shows the format of an IPX packet encapsulated for
transmission over an IP tunnel.
Figure 124 Format of an IPX packet encapsulated for transmission over an IP tunnel
These are the involved terms:
■ Payload: Packet that needs to be encapsulated and routed.
■ Passenger protocol: Protocol that the payload packet uses, IPX in the example.
■ Encapsulation or carrier protocol: Protocol used to encapsulate the payload
packet, that is, GRE.
■ Delivery or transport protocol: Protocol used to encapsulate the GRE packet
and to forward the resulting packet to the other end of the tunnel, IP in this
example.
Decapsulation process
Decapsulation is the reverse process of encapsulation:
1 Upon receiving an IP packet from the tunnel interface, Router B checks the
destination address.
2 If the destination is itself, Router B strips off the IP header of the packet and
submits the resulting packet to the GRE module.
Delivery header
˄Transport protocol˅
Payload header
(Passenger portocol)
GRE header
(Encapsulation protocol)
GRE headerIP header IPX payload
Passenger protocol
Encapsulation protocol
Transport protocol