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 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 ID2. A lower path cost to the Root3. A lower Sending Bridge ID4. 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)