This attribute describes the format of a port identifier string. Objects of this type are always used with an associated LldpPortIdSubtype object, which identifies the format of the particular LldpPortId object instance. If the associated LldpPortIdSubtype object has a value of 'interfaceAlias(1)', then the octet string identifies a particular instance of the ifAlias object (defined in IETF RFC 2863). If the particular ifAlias object does not contain any values, another port identifier type should be used. If the associated LldpPortIdSubtype object has a value of 'portComponent(2)', then the octet string identifies a particular instance of the entPhysicalAlias object (defined in IETF RFC 2737) for a port or backplane component. If the associated LldpPortIdSubtype object has a value of 'macAddress(3)', then this string identifies a particular unicast source address (encoded in network byte order and IEEE 802.3 canonical bit order) associated with the port (IEEE Std 802-2001). If the associated LldpPortIdSubtype object has a value of 'networkAddress(4)', then this string identifies a network address associated with the port. The first octet contains the IANA AddressFamilyNumbers enumeration value for the specific address type, and octets 2 through N contain the networkAddress address value in network byte order. If the associated LldpPortIdSubtype object has a value of 'interfaceName(5)', then the octet string identifies a particular instance of the ifName object (defined in IETF RFC 2863). If the particular ifName object does not contain any values, another port identifier type should be used. If the associated LldpPortIdSubtype object has a value of 'agentCircuitId(6)', then this string identifies an agent-local identifier of the circuit (defined in RFC 3046). If the associated LldpPortIdSubtype object has a value of 'local(7)', then this string identifies a locally assigned port ID.