This value, together with tmnxDot1agCfmMepFacilityVlanId and tmnxDot1agCfmMepFacilityType, specifies the port or interface a facility MEP is associated with; it's only applicable to facility MEPs and must be zero when the MEP is instead attached via interface, SDP-binding, or service objects, and can't be changed once set. An admin sets this when creating a facility MEP, used to monitor connectivity at the port or LAG level rather than at a specific service. Facility MEPs are commonly used to monitor the underlying physical or logical transport independently of whatever services ride on top of it.