Transparent bridges and switches have three main functions: learn, forward, and remove loops. Cisco switches use the 802.1D protocol to remove loops in Layer 2 networks. BPDUs are used to discover the topology of the network, elect a root switch, and notify other switches of topology changes. BPDU's hellos are generated every 2 seconds.

STP elects a root switch, which is switch with the lowest bridge ID (priority + MAC address). Each switch chooses the best port to reach the root, called a root port: This is the port with the lowest accumulated path cost to the root. Each segment has one designated port on one switch, which is used to forward traffic to and from the segment. The switch with the lowest accumulated path cost is chosen. ...

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