3Com 3032 Configuration Manual page 640

636 C 44: C
HAPTER ONFIGURING
GRE
which examines the key, checksum or message sequence number. After the GRE
header is removed, the IP message is processed by the IPX protocol in the same
way as an ordinary datagram.
The system receives a datagram to be encapsulated and routed,. The datagram is
first encapsulated in the GRE message so that the datagram is the payload of a
GRE message. Then the datagram is encapsulated in an IP message. The IP layer
forwards the message. The IP protocol that forwards the messages is often called a
delivery protocol or transport protocol.
The form of an encapsulated message is shown in Figure 199:
Figure 199 Encapsulated tunnel message format (Refer to RFC)
Delivery Header
(Transport Protocol)
GRE Header
(Encapsulation Protocol)
Payload Packet
(Passenger Protocol)
For example: The format of IPX transmission message that is encapsulated in an IP
tunnel is as follows:
Figure 200 Format of transmission message in the tunnel.
IP IP GRE GRE IPX IPX
Passenger Protocol Passenger Protocol
Carrier Protocol or Carrier Protocol or
Encapsulation Protocol Encapsulation Protocol
Transport Protocol Transport Protocol