HP VPN Firewall Appliances VPN Configuration Guide

1
Configuring GRE
The term "router" in this document refers to both routers and routing-capable firewalls and firewall
modules.
Overview
Generic Routing Encapsulation (GRE) is a tunneling protocol that can encapsulate multiple network layer
protocols into virtual point-to-point tunnels over an IP network. Packets are encapsulated at one tunnel
end and de-encapsulated at the other tunnel end.
GRE encapsulation format
Figure 1 GRE encapsulation format
As shown in Figure 1, a GRE-tunneled packet comprises the following parts:
• Payload packet—Original packet. The protocol type of the payload packet is called the passenger
protocol.
• GRE header—After GRE receives a payload packet, it adds a GRE header to the payload packet
to change the payload packet to a GRE packet. GRE is called the encapsulation protocol.
• Delivery header—Transport protocol used to transfer the GRE packet. The system adds a transport
protocol header to the GRE packet to deliver it to the tunnel end.
For example, to transfer an IPv6 packet over an IPv4 network through a GRE tunnel, the system
encapsulates the IPv6 packet in the format shown in Figure 2. The passenger protocol is IPv6, the
encapsul
ation protocol is GRE, and the transport protocol is IPv4.
Figure 2 Format of a GRE-encapsulated packet
Depending on the transport protocol, GRE tunnels include the following types: