STP Blocking State

showintshowint Posts: 68Member ■■□□□□□□□□
It's little said about STP Blocking State.



"The Switch Ports will go into a blocking state at the time of election process, when a switch receives a BPDU on a port that indicates a better path to the Root Bridge, and if a port is not a Root Port or a Designated Port."

Spanning Tree Port States, Blocking, Listening, Learning, Forwarding, Disabled



And here some passage from CCNP Switch Simplified is, which says:

- Receives BPDUs and directs them to the system module
- Does not transmit BPDUs received from the system module



And some excerpt from CCNP Switch OCG:

- In the Blocking State... Instead, a port is allowed to receive only BPDUs so that the switch can hear from other neighboring switches.

One more excerpt:

- After a port initializes, it begins in the Blocking State...



I have a question. if after a port initializes, it begins in the Blocking State.. and doesn't send BPDUs (just receive it), how can it participate in Root Bridge Election (without sending BPDUs) ?

Thanks.

Comments

  • SimridSimrid Posts: 327Member
    I believe the RB is elected by the lowest priority - If they are all left as default, the lowest MAC is chosen. Don't take that as gospel but I believe that's it.
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  • Danielh22185Danielh22185 Posts: 1,195Member
    showint wrote: »
    It's little said about STP Blocking State.



    "The Switch Ports will go into a blocking state at the time of election process, when a switch receives a BPDU on a port that indicates a better path to the Root Bridge, and if a port is not a Root Port or a Designated Port."

    Spanning Tree Port States, Blocking, Listening, Learning, Forwarding, Disabled



    And here some passage from CCNP Switch Simplified is, which says:

    - Receives BPDUs and directs them to the system module
    - Does not transmit BPDUs received from the system module



    And some excerpt from CCNP Switch OCG:

    - In the Blocking State... Instead, a port is allowed to receive only BPDUs so that the switch can hear from other neighboring switches.

    One more excerpt:

    - After a port initializes, it begins in the Blocking State...



    I have a question. if after a port initializes, it begins in the Blocking State.. and doesn't send BPDUs (just receive it), how can it participate in Root Bridge Election (without sending BPDUs) ?

    Thanks.


    I'm trying to understand the question fully... but basically if a port is in a blocking state it will not send BPDUs. This state as indicated can receive BPDUs to listen for spanning tree changes and transition the blocking port to forwarding at anytime if the conditions allow for it. The only function a blocking port serves is to break the possible bridging loops by preventing BPDUs from being sent out that port. Blocking is the 1st transitional state and during a switch first power / spanning-tree convergence technically all ports will transition to forwarding until the spanning-tree topology determines the best root paths and blocks the redundant / less desirable ports.
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