Recommendations to decrease STP convergence time

[JamesEdmondsAspall]

Hi,

We have a set of Catalyst 9200 switches we are testing and configuring before live deployment.

Currently, using all default STP settings, convergence time seems to run anywhere between 30 and 50 seconds.

Are there any recommendations for things to change from the default config, to improve convergence time and decrease the time taken for alternate paths to come up?

Thanks

James

[balaji.bandi]

depends on the network  size.  most cases -

STP's convergence time is 30 to 50 seconds normal

RSTP convergence time much better, so using will be better outcome.

You can consider using below Trusted ports  :  (check the documentations)

• PortFast
• UplinkFast
• BackboneFast

[Joseph W. Doherty · Accepted solution]

Cisco's default STP, I believe (?), still uses non-rapid STP.  Configure rapid STP.  The latter may converge in just a couple of seconds.

Another suggestion would be to insure your root bridge is in the topology's center.  I.e. assign a switch to root, don't let it just default in selecting the root switch.  (You may also want to assign a switch as the secondary root.)

Balaji suggests using Cisco STP enhancements such as uplinkfast and backbonefast.  These would be helpful with non-rapid STP, but, I recall (?), rapid-STP, basically, has such features already baked into it.

Balaji also suggests using portfast, which most do for edge ports.  Besides such ports joining the STP topology sooner (a double edged sword), it also, I recall (?), cuts back on those ports flooding TCNs.

Lastly, as Cisco's STP is actually per VLAN STP, if you don't have the need for each VLAN to have its own STP topology, consider using MST (which is also "rapid").  It can decrease the STP processing load.

[JamesEdmondsAspall]

I've accepted your answer as the solution as you mentioned the idea of setting the root bridge, which ultimately is what improved my convergence time!

Cheers

James

[smithy14975]

My root bridges are the VLANS on the Distribution Layer.  The DR is MEC meaning two DR's but logically are one.  Portchannels going back to the access layer switches.  I needed to add a vlan to the port channel for a switch.  Added to the DR first all was fine.  Added to the Access layer switch 2nd and the switch went down for 15 seconds.

[Martin L]

you should be using RSTP or MST and not classic STP. 

Regards, ML
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[JamesEdmondsAspall]

Thanks for all the input guys.

My belief from a skim of some of the Cisco docks lead me to believe the default mode in use on these switches was RPVST+ which I believe is essentially the same as RSTP but per VLAN?

The setup is quite small during testing (2x 9200 in a stack, with redundant uplinks to another 2x 9200 stack).

I'll fire it back up and see what mode all the switches are in and adjust to RSTP if required.

Will also try setting one of the stacks as the root bridge and see if that helps.

[JamesEdmondsAspall]

Hi again all,

Looks like the default STP mode is Rapid PVST, so that's all good.

I simplified my test down to two switches with two links between them, and after setting one as the root bridge, the convergence time came down from about 30 seconds to just under 20, or about 3 pings worth, so this looks to be the answer

Thanks for the input.

James

[paul driver]

Hello
Just like to add by default ciscos stp diameter is recommended at 7, meaning stp is expects every switch in the estate to be able to reach any other other switch within 7 hops , now if your network diameter is less than this you can decrease this value and the default stp timers(hello/forward/max) will also decrease accordingly thus will assist in convergence timings.