On which ports are STP BPDUs forwarded?

Spanning Tree Protocol (STP) is used to prevent loops in a network by establishing a point-to-point network topology. Bridge Protocol Data Units (BPDUs) are fundamental in this process; they help in determining the network topology by sharing information about the status of the switches and their connections.

BPDUs are forwarded on designated ports because these are the ports that are in a forwarding state. The designated port is the port on a network segment that has the lowest cost path to the root bridge, meaning it is responsible for forwarding traffic towards the root and receiving traffic from the root. By having BPDUs sent out through designated ports, STP ensures that the network topology is consistently updated and maintained, avoiding loops while allowing for efficient traffic flow.

In contrast, other port types play different roles in the STP process. Root ports are the ports that lead to the root bridge but might not necessarily forward BPDUs since their main function is to carry traffic toward the root. Blocked ports do not forward any frames, including BPDUs, as their state is explicitly set to prevent loops. Alternate ports are in a blocking state until they are needed for failover purposes, and thus, they also do not forward BPDUs.