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400-007 · Question #160

Refer to the exhibit. This network is running legacy STP 802.1d. Assuming "hello_timer" is fixed to 2 seconds, which parameters can be modified to speed up convergence times after single link/node…

The correct answer is C. The max_age and forward delay parameters can be adjusted to speed up STP convergence. In IEEE 802.1d STP, the max_age and forward_delay timers are the adjustable parameters that directly control failure detection speed and port state transition time, allowing faster convergence after a link or node failure.

Designing Network Infrastructure

Question

Refer to the exhibit. This network is running legacy STP 802.1d. Assuming "hello_timer" is fixed to 2 seconds, which parameters can be modified to speed up convergence times after single link/node failure?

Exhibit

400-007 question #160 exhibit

Options

  • AThe transit_delay=5 and dpdu_delay=20 are recommended values, considering helto_timer=2
  • BOnly the maximum_transmission_halt_delay and diameter parameters are configurable
  • CThe max_age and forward delay parameters can be adjusted to speed up STP convergence
  • DOnly the transit_delay and bpdu_delay timers are configurable parameters in 802.1d to speed up

How the community answered

(36 responses)
  • A
    6% (2)
  • B
    3% (1)
  • C
    92% (33)

Why each option

In IEEE 802.1d STP, the max_age and forward_delay timers are the adjustable parameters that directly control failure detection speed and port state transition time, allowing faster convergence after a link or node failure.

AThe transit_delay=5 and dpdu_delay=20 are recommended values, considering helto_timer=2

transit_delay and dpdu_delay (bpdu_delay) are not recognized 802.1d STP timer parameters, and the values cited do not map to any valid STP convergence configuration.

BOnly the maximum_transmission_halt_delay and diameter parameters are configurable

maximum_transmission_halt_delay is not a standard 802.1d STP parameter; network diameter is a design input used to calculate recommended timer values rather than a directly configurable convergence timer.

CThe max_age and forward delay parameters can be adjusted to speed up STP convergenceCorrect

Reducing max_age (default 20 s) shortens how long a non-root bridge waits before declaring a superior BPDU lost and initiating a new topology calculation, accelerating failure detection. Reducing forward_delay (default 15 s) decreases the time ports spend in the Listening and Learning states before transitioning to Forwarding, directly cutting the total reconvergence time after a topology change event.

DOnly the transit_delay and bpdu_delay timers are configurable parameters in 802.1d to speed up

transit_delay and bpdu_delay are not standard IEEE 802.1d STP configurable timer names; the correct convergence timers are max_age and forward_delay.

Concept tested: IEEE 802.1d STP convergence timer tuning

Source: https://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/19120-122.html

Topics

#STP 802.1d#convergence timers#max_age#forward delay

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