UNDERSTANDING AND TROUBLESHOOTING THE SPANNIN...

UNDERSTANDING AND TROUBLESHOOTING THE SPANNING TREE PROTOCOLwhen the spanning-tree algorithm determines a port should be transitioned to the forwarding state, the following occurs:

    1?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:

    2?the port waits for the expiration of the forward delay timer. it then moves to the learning state.

    3?in the learning state, the port learns station location information from the source address of packets and adds this information to its forwarding database.

    4?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:

    1?discards packets received from the network segment to which it is attached.

    2?discards packets sent from another port on the switch for forwarding.

    3?does not add addresses to its forwarding database

    4?receives bpdus and directs them to the cpu.

    5?does not transmit bpdus received from the cpu.

    6?receives and responds to network management messages.

    listening state

    the listening state is the first transition for a port from the blocking state. listening is an opportunity for the switch to receive bpdus that may tell the switch that the port should not continue to transition to the forwarding state, but should return to the blocking state (that is, a different port is a better choice).

    there is no address learning or packet forwarding from a port in the listening state.

    a port in the listening state does the following:

    1?discards frames received from the network segment to which it is attached.

    2?discards packets sent from another port on the switch for forwarding.

    3?does not add addresses to its forwarding database

    4?receives bpdus and directs them to the cpu.

    5?processes bpdus received from the cpu.

    6?receives and responds to network management messages.

   

    learning state

    a port in the learning state prepares to participate in frame forwarding. the port enters the learning state from the listening state.

    a port in the learning state does the following:

    1?discards frames received from the network segment to which it is attached.

    2?discards packets sent from another port on the switch for forwarding.

    3?adds addresses to its forwarding database.

    4?receives bpdus and directs them to the cpu.

    5?processes and transmits bpdus received from the cpu.

    6?receives and responds to network management messages.

   

    forwarding state

    a port in the forwarding state forwards packets. the port enters the forwarding state from the learning state when the forward delay timer expires.

    a port in the forwarding state does the following:

    1?forwards packets received from the network segment to which it is attached.

    2?forwards packets sent from another port on the switch for forwarding.

    3?incorporates station location information into its address database.

    4?receives bpdus and directs them to the system cpu.

    5?receives and responds to network management messages.

   

    disabled state

    a port in the disabled state does not participate in frame forwarding or stp. a port in the disabled state is virtually non-operational.

    a disabled port does the following:

    1?discards packets received from the network segment to which it is attached.

    2?discards packets sent from another port on the switch for forwarding.

    3?does not add addresses to its forwarding database.

    4?receives bpdus, but does not direct them to the system cpu.

    5?does not receive bpdus for transmission from the system cpu.

    6?receives and responds to network management messages.

   

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