Refer to the exhibit. Which GRE tunnel configuration command is missing on R2?

While `192.181.2` could potentially be a valid source IP for R2, given the correct answer, it is not the intended missing command or IP.

A network address like `172.16.1.0` cannot be used as a tunnel source; it must be a specific IP address assigned to a physical interface.

The `tunnel source` command is essential for a GRE tunnel and specifies the IP address of the physical interface on R2 that acts as the tunnel's source, meaning the external IP address from which the GRE packets will originate.

The `tunnel destination` command specifies the IP address of the remote router's physical interface (the tunnel's endpoint), not the local router's source IP.