Network Services and Security →mediumMultiple ChoiceObjective-mapped

CCNA Network Services and Security Practice Question

Exhibit

show spanning-tree summary
Switch is in pvst mode
Root bridge for: VLAN0001 VLAN0010 VLAN0020
Extended system ID is enabled
Portfast Default is disabled

show spanning-tree vlan 40
Spanning tree enabled protocol ieee
Root ID    Priority    327...

A switch stack is running PVST+. Users on VLAN 40 lose connectivity for roughly 30 seconds every time the uplink on SW2 flaps. Based on the exhibit, which change would most directly improve convergence for this VLAN?

⚠ Common exam trap

A frequent exam trap is selecting options that change the root bridge election or port roles, such as increasing bridge priority, assuming this will speed up convergence. While root bridge placement affects path selection, it does not reduce the inherent delay caused by legacy STP timers. Another trap is disabling PortFast on access ports, which only affects edge port transitions and does not influence uplink link flap recovery times. Additionally, converting uplinks to routed ports changes the network design and removes STP from those ports, which is not the intended solution for VLAN-specific STP convergence delays. The key mistake is confusing topology optimization with protocol speed improvements.

Answer choices

Why each option matters

Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.

Correct answer & explanation

✓

Configure spanning-tree mode rapid-pvst.

The output shows VLAN 40 is still using the legacy IEEE STP process, which converges much more slowly than Rapid PVST+. Moving the switch to rapid-pvst mode gives VLAN 40 the faster proposal/agreement behavior that typically cuts convergence time from tens of seconds to a few seconds.

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

✓
Configure spanning-tree mode rapid-pvst.
Why this is correct
Configuring `spanning-tree mode rapid-pvst` directly addresses the 30-second connectivity loss by enabling Rapid PVST+ (R-PVST+). PVST+ utilises fixed listening and learning states, contributing to the observed delay during topology changes. R-PVST+ employs a rapid convergence mechanism, such as the Proposal/Agreement (P/A) process, allowing designated and root ports to transition to forwarding much faster, often within sub-seconds. This significantly reduces the reconvergence time for VLAN 40 when SW2's uplink flaps, directly improving user connectivity.
✗
Increase the bridge priority on SW2 for VLAN 40.
Why it's wrong here
Increasing the bridge priority on SW2 for VLAN 40 would assign a higher numeric priority value, making it less likely to become the root bridge for that VLAN. However, the root bridge election only determines the spanning-tree topology; it does not alter the fixed forward-delay timers of PVST+ that cause the 30-second reconvergence. Even if SW2 were the root, PVST+ still requires listening and learning delays, whereas Rapid PVST+ eliminates that delay via Proposal/Agreement. The correct fix is to switch the entire stack to Rapid PVST+, not to manipulate root priority.
When this WOULD be correct
In a different scenario where the question asks about optimizing the spanning tree topology for a network with multiple VLANs and where SW2 is not the root bridge, increasing the bridge priority could help in making SW2 the root bridge, potentially improving overall network performance.
✗
Disable PortFast on all access ports.
Why it's wrong here
Disabling PortFast on all access ports would force every host-facing port to spend 30 seconds in listening and learning states after any link event, making client connectivity worse. PortFast is designed only for edge ports and has no effect on the convergence of an inter-switch trunk when an uplink flaps. The 30-second outage on VLAN 40 stems from the root port transition on SW2's trunk, not from access port behavior, so this change would be counterproductive.
When this WOULD be correct
In a scenario where a question asks about improving convergence time on access ports that are incorrectly configured with PortFast enabled, disabling PortFast could be the correct answer. This would apply if the exam context indicated that the access ports were causing issues due to improper handling of STP states.
✗
Convert the uplink to a routed port.
Why it's wrong here
Converting the uplink to a routed port would eliminate Layer 2 spanning tree on that segment, but it would also terminate the VLAN 40 broadcast domain and require IP addressing on the links. This fundamentally changes the network design and does not address the STP reconvergence delay that occurs when the Layer 2 uplink flaps. The exhibit clearly indicates an STP mode mismatch, not a need for Layer 3 switching, so this is not a valid corrective action.
When this WOULD be correct
In a scenario where the question asks how to optimize routing performance for inter-VLAN traffic and reduce latency, converting the uplink to a routed port could be the correct answer. This would be appropriate if the focus was on routing efficiency rather than spanning tree convergence.

Option-by-option analysis

Why each answer is right or wrong

Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The 200-301 exam frequently reuses these exact scenarios with slightly different constraints.

✓Configure spanning-tree mode rapid-pvst.Correct answer▾

Why this is correct

Configuring `spanning-tree mode rapid-pvst` directly addresses the 30-second connectivity loss by enabling Rapid PVST+ (R-PVST+). PVST+ utilises fixed listening and learning states, contributing to the observed delay during topology changes. R-PVST+ employs a rapid convergence mechanism, such as the Proposal/Agreement (P/A) process, allowing designated and root ports to transition to forwarding much faster, often within sub-seconds. This significantly reduces the reconvergence time for VLAN 40 when SW2's uplink flaps, directly improving user connectivity.

✗Increase the bridge priority on SW2 for VLAN 40.Wrong answer — click to see why▾

Why this is wrong here

Increasing the bridge priority on SW2 for VLAN 40 would make SW2 more likely to become the root bridge, but it does not change the STP convergence mechanism. The 30-second delay is due to the STP mode (802.1D vs Rapid PVST+), not the root bridge location.

★ When this WOULD be the correct answer

In a different scenario where the question asks about optimizing the spanning tree topology for a network with multiple VLANs and where SW2 is not the root bridge, increasing the bridge priority could help in making SW2 the root bridge, potentially improving overall network performance.

Why candidates choose this

Students may think that making SW2 the root bridge would speed up convergence because the root bridge is central to STP operations. However, root bridge election does not affect the convergence time of individual link flaps; the STP mode is the key factor.

✗Disable PortFast on all access ports.Wrong answer — click to see why▾

Why this is wrong here

Disabling PortFast on access ports would actually increase convergence time for those ports, as PortFast allows edge ports to transition to forwarding immediately. The issue is with the uplink flap, which is not an access port, so disabling PortFast would not help and could worsen convergence for end devices.

★ When this WOULD be the correct answer

In a scenario where a question asks about improving convergence time on access ports that are incorrectly configured with PortFast enabled, disabling PortFast could be the correct answer. This would apply if the exam context indicated that the access ports were causing issues due to improper handling of STP states.

Why candidates choose this

Some might confuse PortFast with features that affect uplink convergence, or think that disabling unnecessary features could improve stability. However, PortFast is designed for access ports and does not impact uplink STP convergence.

✗Convert the uplink to a routed port.Wrong answer — click to see why▾

Why this is wrong here

Converting the uplink to a routed port would remove it from STP entirely, which could cause loops if the uplink is part of a redundant topology. The exhibit shows a switch stack with VLANs, and the uplink is likely a trunk; converting to a routed port would break VLAN trunking and is not a direct fix for the STP convergence issue.

★ When this WOULD be the correct answer

In a scenario where the question asks how to optimize routing performance for inter-VLAN traffic and reduce latency, converting the uplink to a routed port could be the correct answer. This would be appropriate if the focus was on routing efficiency rather than spanning tree convergence.

Why candidates choose this

Routed ports do not participate in STP, so a student might think that eliminating STP on the uplink would avoid convergence delays. However, this ignores the need for loop prevention in a switched network and the fact that the uplink is a trunk carrying multiple VLANs.

Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”

Visual reference

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CCNA Network Services and Security Practice Question

Exhibit

Why each option matters

Option-by-option breakdown

Why each answer is right or wrong

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