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I have a simple network with two switches redundantly connected to each other and several computers on each switch talking back and forth across the two switches. The switches support RSTP/MSTP.

I want to be able to simulate a link loss between the two switches by disconnecting the active link and benchmark what the actual delay is until RSTP establishes a new topology and frame forwarding resumes but I'm not sure how to go about doing this.

I've tried measuring the Tx and Rx differential voltage to see if I can see a gap when no frames are being forwarded but it's exceptionally hard to trigger on such an event. Anyone have any ideas?

As a side note, the complexity of my network will grow quickly and I'm just trying to establish a good benchmark method from the start.

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  • Have you tried looking at the switch logs? You should be able to see when the interface is disconnected, and when STP has again converged. You shouldn't try to measure a layer-2 function at layer-1.
    – Ron Maupin ♦
    Oct 26 '16 at 21:43
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    What resolution are you trying to achieve? Seconds? milliseconds?microseconds? It affects how you measure.
    – Ron Trunk
    Oct 26 '16 at 23:11
  • @RonTrunk Ultimately, I want to achieve failover time of no more than 1 or 2 milliseconds.
    – David Jiang
    Oct 27 '16 at 20:58
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    RSTP or any other protocol doesn't converge anywhere near that fast. You would need specialized hardware to make that happen.
    – Ron Trunk
    Oct 27 '16 at 21:22
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Aug 15 '17 at 4:32
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I think you could do this using a SPAN port.

  1. Since your switches are redundantly connected, determine which of the two links is the Alternate.
  2. Set up a SPAN session using the Alternate port as a source and the option "encapsulation replicate"
  3. Connect the SPAN destination to your laptop running Wireshark and start a capture
  4. Disconnect the active link causing the Alternate port to become active
  5. Check the delta time for when the downstream switch sends a BPDU with the Agreement bit set signaling that Sync is complete

Documentation from Cisco: http://www.cisco.com/c/en/us/support/docs/lan-switching/spanning-tree-protocol/24062-146.html#converge2

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Ping from an endpoint off switch A to an endpoint off switch B. In windows OS you can say ping 1.2.3.4 -t so the ping keeps going until you stop it.

Once the ping is going, unplug and/or shut the primary uplink and count the number of seconds until the pings begin responding again.

note that you might get different results depending on failure type (admin interface shutdown, unplug cable, physically cut cable, etc)

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