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Bridge Protocol Data Units (BPDUs) are frames that contain information about the spanning tree protocol (STP). A switch sends BPDUs using a unique source MAC address from its origin port to a multicast address with destination MAC (01:80:C2:00:00:00,[1] or 01:00:0C:CC:CC:CD for Cisco proprietary Per VLAN Spanning Tree).[2]
There are two kinds of BPDUs for 802.1D Spanning Tree:[3]
By default the BPDUs are sent every 2 seconds.