H3C Technologies H3C S7500E Series Switches - Manual (Page 165)

15-8
Figure 15-7 Format of an X packet encapsulated for transmission over an IP tunnel
These are the terms involved:
z Payload: Packet that needs to be encapsulated and transmitted.
z Passenger protocol: Protocol that the payload packet uses, IPX in the example.
z Encapsulation or carrier protocol: Protocol used to encapsulate the payload packet, that is, GRE.
z Delivery or transport protocol: Protocol used to encapsulate the GRE packet and then forward the
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 protocol.
3) The GRE protocol checks the key, checksum and sequence number in the packet, and then strips
off the GRE header and submits the payload to the X protocol for forwarding.
Encapsulation and decapsulation processes on both ends of the GRE tunnel and the resulting
increase in data volumes will degrade the forwarding efficiency for the GRE-enabled device to some
extent.
Protocols and Standards
z RFC 1853: IP in IP Tunneling
z RFC 2473: Generic Packet Tunneling in IPv6 Specification
z RFC 2893: Transition Mechanisms for IPv6 Hosts and Routers
z RFC 3056: Connection of IPv6 Domains via IPv4 Clouds
z RFC 4214: Intra-Site Automatic Tunnel Addressing Protocol (ISATAP)
Tunneling Configuration Task List
Complete the following tasks to configure the tunneling feature: