HP ProCurve Secure Router 7203 dl Advanced Management And Configuration Manual page 905

Using the Web Browser Interface for Advanced Configuration Tasks
To understand the difference between the tunnel's IP address, its source,
and its destination, you should understand how GRE encapsulates traffic.
When the router forwards a packet out a GRE tunnel, it first adds a GRE
header, which can encapsulate all protocols that Ethernet can encapsu-
late. It next encapsulates the GRE frame with a delivery IP header. The
source address in the delivery IP header is the tunnel's source; the
destination address in the delivery IP header is the tunnel's destination.
The tunnel's IP address determines the traffic carried over the tunnel. For
example, if you want to send routing updates over the tunnel, you enable
the routing protocol on the tunnel network.
4. Enter the tunnel's IP address in the IP Address and Subnet Mask fields.
Figure 14-81.Configuring a GRE Tunnel
Setting Up Generic Routing Encapsulation (GRE) Tunnels
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