Refer to the exhibit.
At the end of an RSTP election process, which access layer switch port will assume the discarding
role?
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
Explanation:
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 thatof 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)
In this picture, there are 2 connections and four ports which the question is referencing. You should always remember that when it comes to STP, only ONE of the FOUR ports will be blocked(redundant connections). The bridge with the higher ID/Priority will not be the one to block any port(s). SWT3 would show both ports Fa0/1 and Fa0/12 as forwarding. The port blocking process happens closest to the bottom. It is the responsibility of the lower switch to block a port. Therefore the port states for SWT 4 would be ;
Fa0/2 – FWD/ Forwarding
Fa0/11 – BLK/ Blocking
Also note that since both ports are connecting to the same switch through the same cost/ethernet medium (Fastethernet) and that the bridge ID / MAC would be the same for both ports headed to the other switch, they are not used as a factor of port blocking. The last criteria that is met as a TIEBREAKER is that the LOWER port will forward and the HIGHER will be placed into blocking.
Fa0/2 is the lower port. Fa0/11 is the higher port.
If you do not understand STP/RSTP/PVST, you will NEED to review/study the topic. STP technologies will make up %21 percent of the exam.
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Agreed
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