8021D

IEEE 802.1D is an IEEE standard that defines the operation of MAC bridges (switches) and the Spanning Tree Protocol used to prevent loops in Ethernet local area networks. The specification lays out how bridges forward and filter frames, how they identify themselves (bridge IDs), and how they communicate topology information to other bridges.

Core concepts in 802.1D include the spanning tree mechanism that creates a loop-free logical tree across network bridges. Bridges exchange Bridge Protocol Data Units (BPDUs) to elect a root bridge, compute the shortest paths to the root, and select root ports and designated ports on each network segment. Any redundant, nonessential links are put into a blocking state to avoid forwarding loops, and they can be reactivated if the topology changes.

The standard also defines port roles, path cost concepts, and the rules for learning and aging MAC addresses within a loop-free topology. It addresses the basic port states, timing parameters, and the method by which topology changes are propagated and stabilized after a disturbance.

History and lineage: the original 802.1D was published in 1990 and has undergone several revisions to clarify and improve the protocol. The 2004 revision updated many aspects of the standard. In practice, networks may use related technologies for faster convergence, such as Rapid Spanning Tree Protocol (802.1w) or Multi-Spanning Tree (802.1s) in VLAN environments, but 802.1D remains the foundational standard for basic bridging and Spanning Tree operation.

Significance: 802.1D is widely implemented in Ethernet switches, providing the essential mechanism to prevent switching loops and to maintain a stable, loop-free network topology.