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Question No : 9

Refer to the exhibit. Each of these four switches has been configured with a hostname, as well as being configured to run RSTP. No other configuration changes have been made. Which three of these show the correct RSTP port roles for the indicated switches and interfaces? (Choose three.)

A. SwitchA, Fa0/2, designated
B. SwitchA, Fa0/1, root
C. SwitchB, Gi0/2, root
D. SwitchB, Gi0/1, designated
E. SwitchC, Fa0/2, root
F. SwitchD, Gi0/2, root

Answer: A,B,F
Explanation:
The question says "no other configuration changes have been made" so we can understand
these switches have the same bridge priority. SwitchC has lowest MAC address so, it will become
root bridge and 2 of its ports (Fa0/1 & Fa0/2) will be designated ports (DP). Because SwitchC is
the root bridge the 2 ports nearest SwitchC on SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root
ports (RP) -> B and F are correct.
SwitchB must have a root port so which port will it choose? To answer this question we need to
know about STP cost and port cost.
In general, "cost" is calculated based on bandwidth of the link. The higher the bandwidth on a
link, the lower the value of its cost. Below are the cost values you should memorize:
Link speed Cost SwitchB will choose the interface with lower cost to the root bridge as the root
port so we must calculate the cost on interface Gi0/1 & Gi0/2 of SwitchB to the root bridge. This
can be calculated from the "cost to the root bridge" of each switch because a switch always
advertises its cost to the root bridge in its BPDU. The receiving switch will add its local port cost
value to the cost in the BPDU.
SwitchC advertises its cost to the root bridge with a value of 0. Switch D adds 4 (the cost value of
1Gbps link) and advertises this value (4) to SwitchB. SwitchB adds another 4 and learns that it
can reach SwitchC via Gi0/1 port with a total cost of 8. The same process happens for SwitchA
and SwitchB learns that it can reach SwitchC via Gi0/2 with a total cost of 23 -> Switch B chooses
Gi0/1 as its root port.
Now our last task is to identify the port roles of the ports between SwitchA & SwitchB. It is rather
easy as the MAC address of SwitchA is lower than that of SwitchB so Fa0/2 of SwitchA will be

designated port while Gi0/2 of SwitchB will be alternative port.