802.1D
termStands for: IEEE 802.1D MAC bridges, the original Spanning Tree Protocol
The IEEE standard for MAC bridges, which introduced the Spanning Tree Protocol.
802.1D is the document that makes an Ethernet network with redundant links survivable: without it a loop floods the segment until nothing passes. The original convergence behaviour was slow enough that it became its own class of outage, which is why 802.1w rapid reconvergence and later shortest-path bridging exist. Most engineers meet 802.1D through its failure mode rather than through its text.
Also known as: IEEE 802.1D, 802.1d, MAC bridges, STP standard