Which command is necessary to enable IS-IS on an interface?

To enable IS-IS (Intermediate System to Intermediate System) on an interface, the command must activate the IS-IS routing protocol at the interface level. The correct command effectively tells the router to use IS-IS for routing decisions on that specific interface.

The option that states "isis enable 100" properly invokes the IS-IS process on the interface, with '100' typically representing the area or IS-IS level parameter that the interface will be part of. This command directly places the interface into the IS-IS routing domain, allowing it to advertise and learn routing information within the specified area.

The other choices fail to correctly format the command or place IS-IS in the necessary context. For example, "ip router isis 100" and "router isis 100" are commands that initiate IS-IS globally on the router rather than specifically for an interface. Similarly, "route isis enable 100" does not align with any standard command syntax used in networking for enabling IS-IS. Thus, "isis enable 100" is the appropriate command to achieve the desired outcome.