In order to configure a GRE tunnel interface in the default mode, after assigning an IPv4 address and sourcing the tunnel from an Ethernet interface, it is required to specify the tunnel destination. This is done using the command (config-if)#tunnel destination <ip address>. The tunnel destination is the IP address of the remote end of the GRE tunnel. Without this configuration, the GRE tunnel interface will not be operational as the local router will not know where to send the encapsulated packets.