“…In order to support TRILL OAM functions on sections, as described in [RFC6905], while maintaining the simplicity of a single TRILL OAM Maintenance Domain, the TRILL OAM layer may be implemented on a virtual port with no physical layer (Null PHY). In this case, the Down MEP function is not supported, since the virtual port does not attach to a link; as such, a Down MEP on a virtual port would not be capable of sending or receiving OAM messages.…”