» STP Questions

Here you will find answers to STP Questions

If you are not sure about STP, please read my Spanning Tree Protocol tutorial first.

Question 1


Which switch will be chosen as root bridge and why?

A. Sw-B, because it has the highest MAC address
B. Sw-A, because it has the lowest MAC address
C. Sw-C, because it has the lowest priority
D. Sw-D, because it has the highest priority

Answer: C


The bridge with the lowest bridge ID will become the root bridge. The bridge ID = Priority + MAC so the lowest priority will be chosen first. If all switches’ priorities all equal then their MAC will be compared (the lowest is the best).

Question 2


Spanning Tree Protocol has created a loop-free logical topology in the network that is pictured. How many ports have been placed in the blocking mode?

A. two
B. none
C. three
D. one
E. four

Answer: A


The best way to solve this question is to suppose that one of these switches is the root switch (bridge). For example, if we choose S2 the root bridge and all of its ports will be in forwarding state (so that they are not blocked).

Also suppose that S1 has higher priority than S3 -> the port on S3 connected to S1 will be blocked.


The most difficult problem is that how can we know which port on S4 will be in blocking state. Well, it depends on the Sending Port ID on S4 (Port ID = port priority + port index). If using default values, the default port priority’s value is the same, so S4 will compare port index values, which are unique to each port on the switch. One more time, we suppose S4 connects to S1 via Fa0/1 and S4 connects to S3 via Fa0/2. Because Fa0/2 is inferior to Fa0/1, S4 will select the port connected with Fa0/1 as its root port and block the other port.


Question 3


What is the way that Spanning Tree Protocol prevents switching loops based on the exhibit provided?

A. STP shuts down S3 to eliminate the switching loops.
B. STP identifies cables that must be disconnected to eliminate the redundant paths.
C. STP load balances traffic evenly across the redundant links.
D. STP places ports into the blocking state to disable the redundant paths.
E. STP allows the switches to communicate with a router that makes the traffic forwarding decisions.

Answer: D

Question 4

The following network was designed to provide reliability through redundancy.


Both of the IDF switches, S3 and S4, are in connection with both of the MDF switches, S1 and S2. A loop-free switching environment is nedded, which configuration scenario will achieve this?

A. Spanning Tree Protocol should be running on all switches.
B. Spanning Tree Protocol should be running only on the root bridge.
C. Spanning Tree Protocol is not needed in this network.
D. Spanning Tree Protocol should be running on only the MDF switches S1 and S2.

Answer: A

Question 5

Which two values are used by Spanning Tree Protocol to elect a root bridge? (Choose two)

A. Amount of RAM
B. Bridge priority
C. IOS version
D. IP address
E. MAC address
F. Speed of the links

Answer: B E

Question 6

Which two of these statements regarding RSTP are correct? (Choose two)

A. RSTP cannot operate with PVST+.
B. RSTP defines new port roles.
C. RSTP defines no new port states.
D. RSTP is a proprietary implementation of IEEE 802.1 D STP.
E. RSTP is compatible with the original IEEE 802.1 D STP.

Answer: B E

Question 7

Which switch would STP choose to become the root bridge in the selection process?

A. 32768: 11-22-33-44-55-66
B. 32768: 22-33-44-55-66-77
C. 32769:11-22-33-44-55-65
D. 32769: 22-33-44-55-66-78

Answer: A


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