H3C Technologies H3C S12500 Series Switches - Manual (Page 187)

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GRE encapsulation and de-encapsulation
Figure 73 X protocol networks interconnected through a GRE tunnel
The following takes the network shown in
812H
Figure 73 as an example to describe how an X protocol packet
traverses an IP network through a GRE tunnel:
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Encapsulation process
1. After receiving an X protocol packet from the interface connected to Group 1, Device A submits it
to the X protocol for processing.
2. The X protocol checks the destination address field in the packet header to determine how to route
the packet.
3. If the packet must be tunneled to reach its destination, Device A sends it to the GRE tunnel interface.
4. Upon receiving the packet, the tunnel interface encapsulates the packet with GRE and then with IP.
5. Device A looks up the routing table according to the destination address in the IP header and
forwards the IP packet.
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De-encapsulation process
De-encapsulation is the reverse of the encapsulation process:
1. Upon receiving an IP packet from the tunnel interface, Device B checks the destination address.
2. If the destination is itself and the protocol number in the IP header is 47 (the protocol number for
GRE), Device B removes the IP header of the packet and submits the resulting packet to GRE for
processing (such as checking the GRE key, checksum, and sequence number in the packet).
3. After GRE finishes the processing, Device B removes the GRE header and submits the payload to
the X protocol for forwarding.
NOTE:
GRE encapsulation and de-encapsulation can decrease the forwarding efficiency of tunnel-end devices.
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GRE application scenarios
The following shows typical GRE application scenarios: