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At the end of an RSTP election process, which port and on which Access Layer Switch will assume the discarding

At the end of an RSTP election process, which port and on which Access Layer Switch will assume the discarding role?

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A.
Switch3, port fa0/1

B.
Switch3, port fa0/12

C.
Switch4, port fa0/11

D.
Switch4, port fa0/2

E.
Switch3, port Gi0/1

F.
Switch3, port Gi0/2

3 Comments on “At the end of an RSTP election process, which port and on which Access Layer Switch will assume the discarding

  1. In this question, we only care about the Access Layer switches (Switch3 & 4). Switch 3 has a lower bridge ID than Switch 4 (because the MAC of Switch3 is smaller than that of Switch4) so both ports of Switch3 will be in forwarding state. The alternative port will surely belong to Switch4.

    Switch4 will need to block one of its ports to avoid a bridging loop between the two switches. But how does Switch4 select its blocked port? Well, the answer is based on the BPDUs it receives from Switch3. A BPDU is superior than another if it has:

    1. A lower Root Bridge ID
    2. A lower path cost to the Root
    3. A lower Sending Bridge ID
    4. A lower Sending Port ID

    These four parameters are examined in order. In this specific case, all the BPDUs sent by Switch3 have the same Root Bridge ID, the same path cost to the Root and the same Sending Bridge ID. The only parameter left to select the best one is the Sending Port ID (Port ID = port priority + port index). In this case the port priorities are equal because they use the default value, so Switch4 will compare port index values, which are unique to each port on the switch, and because Fa0/12 is inferior to Fa0/1, Switch4 will select the port connected with Fa0/1 (of Switch3) as its root port and block the other port -> Port fa0/11 of Switch4 will be blocked (discarding role).




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    1. token says:

      I find the question a bit confusing. What about gi0/2 on SW3, what state will it be in? Why is it not even in the question of being discarding? Since gi0/1 is the root port, and the gi0/x port on SW2 should be the designated port for that segment, then gi0/2 should be discarding. Where am I getting it wrong?




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