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
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.
Point worth mentioning although the result/outcome in this case is the same.
“Switch D adds 4 (the cost value of 1Gbps link) and advertises this value (4) to SwitchB.”
only one end is 1Gb, the other end is FE, no will negotiate down to a cost of 19, not 4.
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The question states that no other configuration changes have been made. We can safely assume that all the switches shown in the diagram have the same default BRIDGE ID of 32768. Therefore, now we look at the MAC address in order to determine which switch will become the ROOT BRIDGE.
SWT C has the lowest MAC address and will be elected the root bridge.
Remember this about STP ports ;
A ROOT port is a port which LEADS to the ROOT. A root bridge will NEVER have a ROOT port.
A DESIGNATED port is selected as the method to reach the rest of the layer 2 topology.
Since SWT D’s Gi0/2 is directly connected to the root bridge, this will become a root port.
Since SWT A’s Fa0/1 is directly connected to the root bridge, this will become a root port.
Since SWT A has a lower MAC address than SWT D, SWT A will be deemed the more reliable switch. SWT A’s Fa0/2 will become the designated port to reach the rest of the network.
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You are right again
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