正解:C
To connect R1 to R3 via a GRE tunnel, the configuration must include creating a tunnel interface on both routers, specifying the tunnel source and destination, and setting the IP address of the tunnel interfaces to be in the same subnet. The tunnel source should be the outgoing interface of R1 that connects to the network leading to R3, and the tunnel destination should be the IP address of the interface on R3 that is reachable from R1.
Additionally, routing must be configured to ensure that traffic can reach the tunnel destination.
References:
* Implementing and Operating Cisco Service Provider Network Core Technologies (SPCOR) training materials
* Cisco's official documentation and configuration guides for GRE tunnels