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DES-3326S Layer 3 Fast Ethernet Switch User’s Guide
Understanding and Troubleshooting the Spanning Tree Protocol 319
B
UNDERSTANDING AND
TROUBLESHOOTING THE
SPANNING TREE
PROTOCOL
When the spanning-tree algorithm determines a port should be
transitioned to the forwarding state, the following occurs:
• The port is put into the listening state where it receives
BPDUs and passes them to the switch’s CPU. BPDU
packets from the CPU are processed. If no BPDUs that
suggest the port should go to the blocking state are
received:
• The port waits for the expiration of the forward delay
timer. It then moves to the learning state.
• In the learning state, the port learns station location
information from the source address of packets and
adds this information to its forwarding database.
• The expiration of forwarding delay timer moves the port
to the forwarding state, where both learning and
forwarding are enabled. At this point, packets are
forwarded by the port.