3Com MSR 50 Series Configuration Manual page 1590

1590 C 81: GRE C
HAPTER ONFIGURATION
4 Upon the 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 438 shows the format of an encapsulated packet.
Figure 438 Format of an encapsulated packet
Delivery header
Transport protocol
GRE header
(Encapsulation protocol)
Payload header
(Passenger portocol)
As an example, Figure 439
transmission over an IP tunnel.
Figure 439 Format of an IPX packet encapsulated for transmission over an IP tunnel
IP header GRE header
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.
Depending on the transport protocol, two tunnel modes are present: GRE over
IPv4 and GRE over IPv6.
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.
shows the format of an IPX packet encapsulated for
IPX payload
Passenger protocol
Encapsulation protocol
Transport protocol