portid

Portid refers to an identifier assigned to a network port used by various protocols to reference or identify that port. In most Ethernet networks, a Port ID is a compact value constructed to distinguish a port beyond its physical name, often combining a priority component with a port number.

In Spanning Tree Protocol family deployments (STP, RSTP, MSTP), each bridge port has a Port ID that is included in BPDUs. The Port ID serves as a tie-breaking element during topology convergence, helping determine root ports and designated ports when multiple paths have equal cost. Vendors may encode and display Port IDs in different formats, but the underlying purpose is to uniquely identify ports within the topology.

In Link Layer Discovery Protocol (LLDP), the Port ID appears as part of the Port ID TLV, which communicates the identity of the transmitting port to neighboring devices. The Port ID TLV carries a subtype and a value; the subtype indicates what the Port ID represents—such as an interface name, a MAC address, or a locally assigned string. This enables accurate neighbor discovery and network topology mapping.

Port identifiers are also used in link aggregation protocols such as LACP. In LACP, each port’s Port ID (comprising a priority and a port number) participates in the negotiation and operation of an aggregated link. The Port ID helps coordinate port inclusion and load balancing across members of the bundle.

Overall, portid is a general concept used across networking protocols to uniquely reference a specific port for control, discovery, and management. The exact representation and interpretation depend on the protocol and vendor.