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(Bridge Protocol Data Unit) A spanning tree protocol (STP) message unit that describes the attributes of a switch port such as its MAC address, priority and cost to reach. BPDUs enable switches that participate in a spanning tree protocol to gather information about each other. See spanning tree protocol.

spanning tree protocol

An algorithm that runs in network bridges and switches to prevent loops, in which packets keep going around in circles. In a small network with one or two switches, the spanning tree protocol (STP) has little value; however, in a large network where many switches are connected to each other via bridges, redundant paths can cause loops.

The algorithm creates a hierarchical "tree" that "spans" the entire network. It determines all redundant paths and makes only one of them active at any given time. In addition, if there are redundant paths and one of them fails, it allows the other to take over. The spanning tree protocol is part of the IEEE 802.1 network management standard. See IEEE 802 and BPDU.

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