The root bridge sends configuration BPDUs on all its ports periodically, every 2 seconds by default. (These configuration BPDUs include the STP timers, therefore ensuring that all switches in the network use the same timers.) On each LAN segment the switch that has the designated port forwards the configuration BPDUs to the segment; all switches in the network therefore receive these BPDUs, on their root port.
All ports on a LAN segment that are not root ports or designated ports are called nondesignated ports and transition to the blocking statethey do not send data, so the redundant topology is logically disabled. In Figure 2-4, port 2 on switch X is the nondesignated port, and it is in the blocking state. Blocking ports do, however, listen for BPDUs.
If a failure happensfor example, if a designated port or a root bridge failsthe switches send topology change BPDUs and recalculate the spanning tree. The new spanning tree does not include the failed port or switch, and the ports that were previously blocking might now be in the forwarding state. This is how STP supports the redundancy in a switched network.
Figure 2-5 illustrates the various STP port states.
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