CCNA – RSTP Common Questions
This is an Updated CCNA exam question . You might see a different IP addressing, Port allocation and Configurations.
As usual, take some time to read through the question so as to clearly understand what Cisco want you to do. its always a good thing to read the questions twice or even thrice to have a clear mental picture of what its all about.
Note: This RSTP Questions are used for demonstration only; you might see different IP addressing and Port allocation in the exam. But it all works the same way if you could just grasp the technique.
I suggest you use packet tracer for practice.
Download Cisco Packet tracer here
Why has this switch not been elected the root bridge for VLAN1?
A. It has more than one internee that is connected to the root network segment.
B. It is running RSTP while the elected root bridge is running 802.1d spanning tree.
C. It has a higher MAC address than the elected root bridge.
D. It has a higher bridge ID than the elected root bridge.
As we can see from the output above, the priority of the root bridge is 20481 while that of the local bridge is 32769.
Which command enables RSTP on a switch?
A. spanning-tree mode rapid-pvst
B. spanning-tree uplinkfast
C. spanning-tree backbonefast
D. spanning-tree mode mst
Refer to the exhibit. Which statement is true?
A. The Fa0/11 role confirms that SwitchA is the root bridge for VLAN 20.
B. VLAN 20 is running the Per VLAN Spanning Tree Protocol.
C. The MAC address of the root bridge is 0017.596d.1580.
D. SwitchA is not the root bridge, because not all of the interface roles are designated.
From the output above, Fa0/11 is the root port this shows that switchA is not the root bridge A is not correct.
Also the output shows that switch is running Rapid STP, not PVST. B is wrong answer.
0017.596d.1580 is the MAC address of this switch, not of the root bridge. The MAC address of the root bridge is 0017.596d.2a00. C is out of the question.
All of the interface roles of the root bridge are designated. SwitchA has one Root port and 1 Alternative port so it is not the root bridge D is correct.
Which three statements about RSTP are true? (Choose three)
A. RSTP significantly reduces topology reconvening time after a link failure.
B. RSTP expands the STP port roles by adding the alternate and backup roles.
C. RSTP port states are blocking, discarding, learning, or forwarding.
D. RSTP provides a faster transition to the forwarding state on point-to-point links than STP does.
E. RSTP also uses the STP proposal-agreement sequence.
F. RSTP uses the same timer-based process as STP on point-to-point links.
Answer: A B D
Which two states are the port states when RSTP has converged? (choose two)
Answer: A D
RSTP only has 3 port states that are discarding, learning and forwarding. When RSTP has converged there are only 2 port states left: discarding and forwarding but the options don’t mention about discarding state so blocking state which places A to be the best the best alternative answer.
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 802.1D STP.
E. RSTP is compatible with the original IEEE 802.1D STP.
Answer: B E
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)
Answer: A B F
From the question no other configuration changes have been made; this means that the switches have the same bridge priority.
From the output, since SwitchC is the root bridge then the 2 ports closest to which is SwitchA (Fa0/1) and SwitchD (Gi0/2) will be root ports. B and F are correct.
Now this is where it becomes tricky: Which of the SwitchB port is to be chosen as a root port? This question need you to know about Switch port selection process
Basically, “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
1 Gbps 4
SwitchB will would have to choose the interface with lower cost to the root bridge as the root port, so you must calculate the cost on interfaces 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.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. A is correct
See all the port roles of these switches:
SwA: Fa0/2 is a Designated Port (forwarding state) and Fa0/1 is a root port (forwarding state)
SwB: Gi0/2 is an Alternative port (blocking State)Gi0/1 is a root port (forwarding state)
SwC: Fa0/1 & Fa0/2 are designated Port (forwarding state)
SwD: Gi0/1 is a designated port (forwarding state)Gi0/2 is a root port (forwarding state)
Refer to the exhibit. The output that is shown is generated at a switch. Which three of these statements are true? (Choose three)
A. All ports will be in a state of discarding, learning or forwarding.
B. Thirty VLANs have been configured on this switch.
C. The bridge priority is lower than the default value for spanning tree.
D. All interfaces that are shown are on shared media.
E. All designated ports are in a forwarding state.
F. The switch must be the root bridge for all VLANs on this switch.
Answer: A C E
From the output, we see that all ports are in Designated role (forwarding state) A and E are correct.
The bridge priority of this switch is 24607 which is lower than the default value bridge priority 32768 C is correct.
Refer to the exhibit. At the end of an RSTP election process, which access layer switch port will assume the discarding role?
In this question, we are only focusing on 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 because it receives lower BPDUs from Switch3.