STP Network Design and Implementation 3 — Questions and Answers
Question 1: A network designer wants per-VLAN load balancing across redundant uplinks between access and distribution switches. Which STP variant best supports this design?
- PVST+ (Correct answer)
- STP (802.1D)
- MSTP (802.1s)
- RSTP (802.1w)
Correct answer: PVST+
PVST+ (Per-VLAN Spanning Tree Plus) runs a separate STP instance per VLAN, enabling different VLANs to use different root bridges and achieve uplink load balancing.
Question 2: In an MSTP design, what is the purpose of a Common and Internal Spanning Tree (CIST)?
- It interconnects multiple MST regions and interoperates with 802.1D bridges (Correct answer)
- It provides per-VLAN spanning tree for legacy switches
- It replaces PVST+ in Cisco proprietary implementations
- It acts as a backup to the IST inside each MST region
Correct answer: It interconnects multiple MST regions and interoperates with 802.1D bridges
The CIST spans all switches in all MST regions and interoperates with 802.1D/PVST+ bridges, forming the backbone of MSTP topology.
Question 3: A network engineer notices that STP reconverges frequently due to topology changes triggered by end-device ports. Which feature directly reduces unnecessary TCN propagation?
- PortFast on access ports (Correct answer)
- Root Guard on uplinks
- BPDU Guard globally
- Loop Guard on trunk ports
Correct answer: PortFast on access ports
PortFast suppresses TCN BPDUs when ports transition, because end-device ports should not cause topology changes that flush MAC tables network-wide.
Question 4: In a network with dual-homed access switches, which STP port role describes the non-root, non-forwarding port that provides a redundant path to the root bridge?
- Alternate port (RSTP) (Correct answer)
- Backup port (RSTP)
- Non-designated port (STP)
- Blocked port (STP)
Correct answer: Alternate port (RSTP)
In RSTP, the Alternate port is a non-forwarding port on a switch that has a better path to the root through a different bridge than its current root port.
Question 5: A data center design uses a collapsed core/distribution model with two core switches. To optimize STP, the engineer sets one core switch as the primary root and the other as the secondary root. What command sets the secondary root on Cisco IOS?
- spanning-tree vlan X root secondary (Correct answer)
- spanning-tree vlan X priority 4096
- spanning-tree vlan X root backup
- spanning-tree vlan X secondary enable
Correct answer: spanning-tree vlan X root secondary
The 'spanning-tree vlan X root secondary' command sets the bridge priority to 28672, making it the backup root if the primary root fails.
Question 6: Which STP design consideration is most critical when connecting a new switch to an existing production network to prevent a root bridge election disruption?
- Pre-configure the bridge priority to a higher value than the existing root (Correct answer)
- Enable BPDU Filter on all ports of the new switch
- Set all ports to PortFast before connecting
- Use a lower MAC address switch to win the election
Correct answer: Pre-configure the bridge priority to a higher value than the existing root
Setting a higher bridge priority on the new switch (e.g., 61440) ensures it cannot win a root election and disrupt the existing topology.
Question 7: In a multi-VLAN environment using PVST+, what is the operational impact of having too many VLANs each running their own STP instance?
- Excessive CPU usage on switches due to processing many STP instances simultaneously (Correct answer)
- Faster convergence because each VLAN converges independently
- Reduced memory usage due to shared BPDU processing
- Automatic load balancing without any configuration
Correct answer: Excessive CPU usage on switches due to processing many STP instances simultaneously
Each PVST+ instance consumes CPU and memory resources; running hundreds of STP instances on a switch can cause CPU spikes and degrade switch performance.
STP Network Design and Implementation 3
A network designer wants per-VLAN load balancing across redundant uplinks between access and distribution switches.
Which STP variant best supports this design?