As shown in the figure, three switches run the MSTP protocol. So how many seconds is the convergence time for the ports connected to SW2 and SW3?

Question

Options

• A30 seconds
• B2 seconds
• C15 seconds
• D10 seconds

• A
  85% (46)
• B
  9% (5)
• C
  4% (2)
• D
  2% (1)

Explanation

Option A (30 seconds) is correct because in MSTP, non-edge ports connecting switches must transition through two timed states before reaching Forwarding: the Listening state (15 seconds) and the Learning state (15 seconds), totaling 30 seconds of convergence time. The ports connecting SW2 and SW3 are inter-switch links, not edge ports, so they undergo this full state machine traversal.

Why the distractors are wrong:

• B (2 seconds) - This is the convergence time for RSTP rapid negotiation on point-to-point edge links; MSTP can use RSTP mechanisms, but only for edge/PortFast-enabled ports, not inter-switch links in this topology.
• C (15 seconds) - This is only one Forward Delay timer; you need two full Forward Delay periods (Listening + Learning) to reach Forwarding.
• D (10 seconds) - Not a standard STP/MSTP timer value at all; this is a distractor with no basis in the protocol specification.

Memory tip: Think "15 + 15 = 30" - two Forward Delay timers stacked back-to-back give you the full STP/MSTP convergence time for non-edge ports. If you see inter-switch links with default timers, always add both delays.

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