GRE Tunnel Key

Hello Lam

The choice of GRE key depends on the routing configured in each router.

There are two GRE tunnels between R1 and R2 with specific IP addresses. The default route within R1 and R2 indicates which should be the next-hop IP. This next-hop IP is also what specifies which tunnel should be used.

R1 has a default route to and R2 has a default route to so for all communication in both directions, Tunnel 1 should be used since tunnel 1 is on the subnet.

Without the GRE key, as you see in the lesson, either tunnel will be used. By putting in the GRE key, only tunnel 1 is used.

Now if you want to use Tunnel 2, you simply change the routing so that the default route of R1 points to and the default route of R2 points to Then traffic will use the other tunnel.

I hope this has been helpful!