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  1. Hi Rene,
    In blocking port state,switch not able to receive and transmit any BPDUs,then How blocking port state move to listening state.


  2. Mohammad,
    The concept of an Alternate port was introduced with Rapid Spanning Tree. This feature takes over what the traditional (802.1 D) spanning-tree enhancement of “uplink-fast” used to do. The Alternate port serves as a “hot-standby” for a switch’s Root Port, but Alternate Port is considered to be in a Discarding state (Discarding is the RSTP term for Blocking, Listening, and Disabled for spanning-tree).

    This means that an Alternate port can receive BPDUs but will not send them. As soon as a Root Port fails, the Alternate Port will immediately transition to forwarding, skipping the Learning state (there is no such thing as “Listening” in Rapid Spanning Tree).

    You are correct that Root and Designated ports both send and receive BPDUs.

  3. Hello Rene

    In your post you mentioned ports in block mode take the 20+15+15 before forwarding in some cases. Could you tell me in which case cause i was trying to simulate but i only got lis->lear->for states and not Max age time.

    Thanks in advance.

  4. Hello Jose

    There are two situations which require a switch to begin STP convergence procedures. The direct and indirect link failures. Let’s take a look at both:

    1. A direct link failure is when a switch detects a loss of carrier on its own port and immediately declares the port as disconnected. Take a look at the following figure:

    The STP is converged and Port B is in blocking state. Imagine Port A goes down. The bridge no longer detects a carrier on that port and immediately causes Port B to go through the STP procedure going into the Listening and Learning states for a total of 15+15=30 seconds. Convergence time in this case is at 30 seconds.

    1. An indirect link failure is when a link goes down on another switch. This is not immediately detected. Take a look at the following figure:

    In this figure, SW1 is the Root Bridge and Port B on SW3 is the blocked port. Also, assume SW2 has a better Bridge ID than SW3. Imagine that Port C on SW2 goes down. How will Port B on SW3 react? Let’s go through the steps.

    • SW2 will no longer be receiving BPDUs from SW1 and will declare itself root bridge.
    • SW2 will start advertising new BPDUs to SW3 telling SW3 that it is root bridge. SW3 will ignore them because SW1 BPDUs it’s still receiving are superior.
    • SW3 will keep the information from the previously received BPDUs on Port B for 20 seconds which is the blocking timer
    • Once this timer is expired, Port B will begin considering the BPDUs in the Listening state and will begin relaying SW1’s BPDUs to SW2 since they are superior
    • Then SW2 will detect the better information it is receiving on Port D and will cycle the port through Listening and Learning.
    • Both switches (2 and 3) will eventually place their ports in the forwarding states and connectivity will be recovered.

    In this case, the total time is 20+15+15=50.

    The 50 second convergence time is more often quoted for STP because it is the absolute longest time that you may have to wait for STP to fully converge.

    I hope this has been helpful!


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