R21xx-HP FlexFabric 11900 Layer 3 IP Services Configuration Guide

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• GRE over IPv4—The transport protocol is IPv4, and the passenger protocol is any network layer
protocol.
• GRE over IPv6—The transport protocol is IPv6, and the passenger protocol is any network layer
protocol.
GRE encapsulation and de-encapsulation
Figure 85 X protocol networks interconnected through a GRE tunnel
The following takes the network shown in Figure 85 as an example to describe how an X protocol packet
traverses an IP network through a GRE tunnel:
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.
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.
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.
Protocols and standards
• RFC 1701, Generic Routing Encapsulation (GRE)