H3C Technologies H3C S12500-X Series Switches - Manual (Page 172)

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Configuring GRE
Overview
Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate any network layer
protocol (such as IPv6) into a virtual point-to-point tunnel over an IP network (such as an IPv4 network).
Packets are encapsulated at one tunnel end and de-encapsulated at the other tunnel end. The network
layer protocol of the packets before encapsulation and after encapsulation can be the same or different.
GRE encapsulation format
Figure 61 GRE encapsulation format
As shown in Figure 61 , a GRE-tunneled packet includes the following parts:
• Payload packet—Original packet. The protocol type of the payload packet is called the passenger
protocol. The passenger protocol can be any network layer protocol.
• GRE header—Header that is added to the payload packet to change the payload packet to a GRE
packet. A GRE header includes the number of encapsulations, version, passenger protocol type,
checksum, and key. GRE is called the encapsulation protocol.
• Delivery header—Header that is added to the GRE packet to deliver it to the tunnel end. The
transport protocol (or deliver protocol) is the network layer protocol that transfers GRE packets. The
device supports IPv4 and IPv6 as the transport protocols. When the transport protocol is IPv4, the
GRE tunnel is called the "GRE over IPv4 tunnel." When the transport protocol is IPv6, the GRE
tunnel is called the "GRE over IPv6 tunnel."