GRE packets are encapsulated within IP and use IP protocol type 47
A GRE interface definition includes:
+ An IPv4 address on the tunnel
+ A tunnel source
+ A tunnel destination
Below is an example of how to configure a basic GRE tunnel:
|interface Tunnel 0
ip address 10.10.10.1 255.255.255.0
tunnel source fa0/0
tunnel destination 172.16.0.2
In this case the “IPv4 address on the tunnel” is 10.10.10.1/24 and “sourced the tunnel from an Ethernet interface” is the command “tunnel source fa0/0”. Therefore it only needs a tunnel destination, which is 172.16.0.2.
Note: A multiple GRE (mGRE) interface does not require a tunnel destination address.