Switches running STP communicate using Bridge Protocol Data Units (BPDUs). A BPDU is a special frame sent to a multicast address that all STP switches recognize. BPDUs carry information about the sending switch's bridge ID, the root bridge it knows about, and path costs. Switches exchange BPDUs every seconds by default. By comparing BPDUs, switches agree on the network topology and determine which ports to block. Without BPDUs, there would be no way to coordinate loop prevention.
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$ curl repovive.com/roadmaps/network-design/spanning-tree-and-link-aggregation/bridge-protocol-data-units
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