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DES-3326 Layer 3 Fast Ethernet Switch User’s Guide
D
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.
Blocking State
A port in the blocking state does not forward packets. When the switch is booted, a BPDU is sent to
each port in the switch putting these ports into the blocking state. A switch initially assumes it is the
root, and then begins the exchange of BPDUs with other switches. This will determine which switch in
the network is the best choice for the root switch. If there is only one switch on the network, no BPDU
exchange occurs, the forward delay timer expires, and the ports move to the listening state. All STP
enabled ports enter the blocking state following switch boot.
A port in the blocking state does the following:
• Discards packets received from the network segment to which it is attached.
• Discards packets sent from another port on the switch for forwarding.
• Does not add addresses to its forwarding database
• Receives BPDUs and directs them to the CPU.
• Does not transmit BPDUs received from the CPU.
• Receives and responds to network management messages.
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