In RHEL 7.1 or higher, how do you set the gateway for an existing connection called 'lab'?

The command that correctly sets the gateway for an existing connection named 'lab' in RHEL 7.1 or higher is the one that utilizes the nmcli con mod syntax. This command modifies the properties of an existing connection.

When using nmcli, the mod (modify) command allows you to change various aspects of the connection's configuration, including the IPv4 settings such as the gateway. By specifying ipv4.gateway followed by the gateway address, the command effectively updates that specific property of the 'lab' connection without altering other configurations.

This level of granularity is essential for network management, allowing administrators to make specific alterations to network configurations in a controlled manner. The other options, while they may seem similar, do not accurately use the correct syntax or command associated with NetworkManager for modifying an existing connection.

In summary, using nmcli con mod lab ipv4.gateway 172.25.1.254 is the proper way to set or change the gateway within the existing network configuration for the connection named 'lab'.