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Beginner

MSTP CONVERGENCE TIME

HI,

 

 How much time switch will take to MSTP convergence and shift the traffic to alternate path.

 

what is the impact time? black hole?

my network diameter is  : 16

my hello time is : 2s

Forwading time is: 15s

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Beginner

Re: MSTP CONVERGENCE TIME

HI Genius,

 anyone can help to clarify the above querys.

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Hall of Fame Cisco Employee

Re: MSTP CONVERGENCE TIME

Hi Jagadeesan,

It is not possible to answer this question with guaranteed values. However, if you have a single MSTP region, then the MSTP convergence time is quite the same as with RSTP, and you can expect the convergence to be very quick, depending on the nature of the topology change:

  • The loss of a Root port on a switch will be handled instantaneously, as soon as the former Root port goes down.
  • The loss of a Root port on an upstream switch that involves a local Alternate Discarding port toward that upstream switch to become Designated Forwarding may take up to 2x hello timer (4 seconds by default), though typically this will be well under 1 second.
  • The arrival of a new Root port after a link comes up requires a wave of Proposal/Agreement negotiations between pairs of switches. A single Proposal/Agreement is a matter of a few tens of milliseconds.
  • A brain death of a switch is detected within 3x hello time - 6 seconds by default, though a switch that becomes brain-dead is usually going to create a switching loop anyway since it cannot block its own ports. Hopefully, you will never face this scenario.

In a well-designed network, RSTP/MSTP convergence should occur well under 1 second, but the actual mileage may vary depending on the actual design of the network, the extent of the topology change and the intensity of its reoccurrence, and the CPU load on the switches.

My two cents. Please feel welcome to ask further!

Best regards,
Peter

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