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.