Standard STP convergence time?

For regular STP with default timers on Cisco equipment and properly configured ports - it is 30 seconds.

Regards, Andrey.

Hmmm. I would disagree with that.

I too was somewhat confused by observing that re-convergence can take either about 30 seconds or about 45 seconds (maybe 35 and

50?).

I believe that it is like this.

For a port that is not connected that gets connected STP has to go through all of the states

Blocking Listening Learning Forwarding 45-50 seconds - If I recall correctly

For a port that is already in the network and is Blocking The transition to Forwarding is about 30-35 seconds

The default timers will be easy to find on the web.

Note that the times do not really range over the interval indicated but I have specified a range since I cannot remember the correct times and BPDUs are sent only every two seconds so there will be a range of times introduced by the port state change possibly occurring just before or just after a BPDU tx or rx event.

Of course there are other considerations.

For a newly connected port then there may be physical speed/duplex negotiation etherchannel negotiation trunking negotiation

There are also various STP optimisations portfast uplink fast backbone fast

Lastly of course there is Rapid STP which is substantially changed from traditional STP and has much lower convergence times. I have not worked with it but have the idea that convergence times of one to a few seconds are reasonable.

Thanks, I think 50s=3D2x15s(forwarding delay)+20s(max age time) so the max. convergence time is 50s and min. is 30s. right? why does max age time is from 0-20s?

st

That's fine. How about some facts?

Why? 2 x forwarding delay is 28.5 sec. Where are additional 15-20 sec coming from?

So what's the difference between newly connected vs already connected?

Not relevant to original question. While etherchannel/trunking negotiation indeed adds time, it's not part of STP convergence.

That's enabled on edge ports only and doesn't affect convergence time at all.

Cisco extensions and not part of original 802.1D spec.

Again, Rapid STP is completely different beast.

Regards, Andrey.

Andrey,

to sum it up. Going through all stp states takes 30 seconds! Neither less nor more :)

Andre

Let's say the lowest cost root path includes 3 switches between you and the root, and there is also an inferior path to the root with higher cost.

If something should affect the continuity between , say, the root and the first switch closest to the root on your "preferred" path, your switch will not detect the failure until

20 seconds pass without seeing any of the 'preferred' bpdus. After realizing there is a failure (20 seconds), your switch will then go through the normal 15 listening/15 learning second convergence. But that doesn't happen until your switch realizes there is a problem, which too 20 seconds of no-show bpdus up front. Total time - 50 seconds.

My bad. Indirect failure will indeed produce 50 seconds convergence. Convergence for direct failure is 30 sec though.

Regards, Andrey.