User's Manual

Configuring GRE Tunnels
308625-14.00 Rev 00
1-3
How GRE Tunneling Works
A simple point-to-point GRE tunnel terminates at router interfaces at each end of
the tunnel (Figure 1-1
). Each of these interfaces has at least two addresses: a
physical address and one or more logical addresses. The physical address, which
is always an IP address, is visible to the devices making up the intervening
network cloud.
Figure 1-1. Simple GRE Tunnel Components
At each tunnel end point, there is one logical address for each protocol configured
for encapsulation over the tunnel (IP, IPX, or OSI). The logical addresses are not
visible to the devices that make up the intervening network cloud. They are private
addresses, visible only to the networks on either side of the tunnel.
IP0095A
Router
1
Router
2
Local logical
host interface
Local physical
router interface
Remote logical
host interface
Remote physical
router interface
GRE tunnel
Host
A
Host
B