STP Listening State

Experts,

I understand that in Listening state, the Switch would listen to BPDU from other switches to establish its Root and Designated ports. If the root bridge is not selected ( e.g. in situation wherein all switches are powered on at same time) , the root bridge selection would happen in the listening state. Is this correct ?

 

Regards

Ram

Comments

  • yes it is correct, in listening state three things happen:

     

    1) root bridge election.

    2) root port

    3) designated port.

     

    HTH

  • Hi Ram,

       This is more of a academic question, i guess it could be visible on a debug. But since in listening state each bridge does not send BPDU's, i would say until learning state, when bidirectional BPDUs are exchanged, each bridge will consider itlsef as the root. So the root bridge election, in my opinion(considering a link that come up in the same time in both switches) should happen somewhere in the beginning of learning state.

    Good luck with your studies!

  • Christian,

     

    You answer seems to be confusing to me. As Brian said  in his Switch Bootcamp in the listening states switch are Exchanging BPDU, and in learning state switch are build up the mac address table for the respective VLAN.

    Happy Studying..

  • Hi Rajput,

       Yes, i delayed with 1 unit STP states, left over blockign mode; in blocking mode it does not send BPDUs; in listening state it sends and receives BPDUs so all port states are negotiated at this point. Thanks for observation.

    Good luck with your studies!

  • Listening State:

    This is actually the time when the switch is going through the root bridge election, root port, designated bridge and designated port decision process. It is waiting to see if it should go into blocking state. If the forward delay timer expires before it is told to start blocking, it moves into the listening state. Since all ports are brought up initialized to forward, the forward delay time is important to allow a blocking decision to be made before a loop is formed.

    Default foward delay is 15 Secs.

     

    Learning State :

     

    After the decisions are made, the switch moves into the learning state to build up the mac table. The forward delay timer is reset once the port moves into the listening state. 

     

    HTH...

    Deepak Arora

    http://deepakarora1984.blogspot.com

     

  • rajput your explanation is correct.

     

    1. On Listening state the switch exchange the BPDUs and after 15 seconds port moves to learning state( in learning state switch start to build the MAC table).

    2. During the listening state the port discards the frames received from the attached network segment.

     

    HAPPY STUDY

     

    [:D]

    Christian,

     

    You answer seems to be confusing to me. As Brian said  in his Switch Bootcamp in the listening states switch are Exchanging BPDU, and in learning state switch are build up the mac address table for the respective VLAN.

    Happy Studying..

     

  • Thx Deepak.

    Due to mean to say that while in listening state, if the forward delay timer expires, it cotinues to remain in listening state ? We assume that the matter to identify all root ports, designated ports and root bridge would be settled  before the forward delay timer expires. Incase, it does not settle, how does the switch takes care of this ? TIA

     

    If the forward delay timer expires before it is told to start blocking, it moves into the listening state. Since all ports are brought up initialized to forward, the forward delay time is important to allow a blocking decision to be made before a loop is formed.
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