STP is defined in the IEEE 802.1D standard. In order to maintain a loop-free logical topology, every two seconds, switches pass Bridge Protocol Data Units (BPDUs). BPDUs are data messages used within a spanning tree topology to pass information about ports, addresses, priorities, and costs. The BPDUs are tagged with the VLAN ID.
Figure 31.1 below shows how loops can be created in a network. Because each switch learns about VLAN 20, it also advertises to other switches that it can reach VLAN 20. Soon enough, each switch thinks it is the source for VLAN 20 traffic . . .
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