350-401 Practice Question: Is troubleshooting an STP issue in a network that…

Answer analysis

Option-by-option breakdown

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

• ✓

  UplinkFast is enabled, which is incompatible with Rapid PVST+ and causes the switch to use legacy STP convergence.

  Why this is correct

  Correct because UplinkFast is a proprietary Cisco feature designed for legacy 802.1D PVST+ to quickly fail over to a precomputed alternate root port when the primary uplink fails. However, UplinkFast is mutually exclusive with Rapid PVST+/RSTP, and when enabled it forces the switch to fall back to the classic Spanning Tree Protocol algorithm. That legacy mode relies on Max Age (20 seconds) and Forward Delay (15 seconds) timers, so after a root port failure the switch takes 30–50 seconds to converge instead of milliseconds, matching the behavior described.

• ✗

  SW2 has a lower priority than SW1, so it takes longer to become the root bridge after failure.

  Why it's wrong here

  Incorrect because the root bridge election is decided by bridge priority, but the priority value has no effect on convergence timing. Rapid PVST+ uses RSTP's proposal/agreement mechanism to rapidly transition ports to forwarding, and this occurs in milliseconds regardless of which switch is elected root. A lower priority on SW2 would only make it more likely to be root under normal operation; it would never cause it to 'take longer' to become root after a failure, since convergence speed is determined by protocol state machines and timers, not by priority.

• ✗

  BPDU Guard is enabled on the uplink ports, which prevents BPDU exchange.

  Why it's wrong here

  Incorrect because BPDU Guard is a PortFast security feature that error-disables a port the moment it receives any BPDU, to prevent unauthorized switches from participating in spanning tree. It does not filter, drop, or slow BPDU exchange while the link is up; rather, it shuts the port down completely, which would cause an immediate loss of connectivity, not a delay in convergence. On normal uplink ports BPDU Guard is not enabled, and even if it were, it would not alter the STP timers or slow down the root port failover process.

• ✗

  Loop Guard is enabled on the uplink ports, which delays port transition.

  Why it's wrong here

  Incorrect because Loop Guard is designed to protect against unidirectional link failures by blocking a port that stops receiving BPDUs, placing it in a loop-inconsistent state. It does not add any artificial delay to the normal forwarding transition process; instead, it actively prevents a port from becoming designated when BPDUs are absent, which is a safety measure, not a timer-based delay. Loop Guard would only interfere after a BPDU loss and would block the port entirely, so it cannot explain the slower convergence caused by using legacy STP timers.

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