STP timers - convergence

mguy

Hi guys, i'm trying to understand exactly how long STP takes to converge from a failure.

Here are the parameters from my study sources:

Dead Timer: 20 seconds [10 hello BPDUs (each 2 seconds)]
Forward Delay: 15 seconds (transition timer from one state to the next)

So For example, a switch in the topology fails.
So how long exactly does it take for STP to converge?

20 seconds - it learns something is wrong initiates blocking
15 seconds - blocking. initiates listening.
15 seconds - listening. initiates learning.
15 seconds - learning. converged. starts forwarding

total time: 65 seconds

Is this accurate?

oli356

I only have gone over STP once from CBT Nuggests video, his was about 50 seconds as far as I remember. If you can find a video about it which shows a live demo. Sorry, can't really help anymore

With RSTP it was like a matter of seconds.

EV42TMAN

yes i just watched the video oli356 is talking about the original STP takes about a minute and RSTP is a few seconds to auto failover

Somnipotent

Default convergence times for 802.1d = MaxAge (how long the bridge waits after not hearing any hellos before initiating topology change) 20s + Listening 15s + Learning 15s = 50 secs.

Why do you have that extra 15s in there? Once a port does not hear a hello packet, it goes into blocking and immediately enters the listening state. No extra 15s just for blocking.

mguy

STP Timer Settings

Max Age = 20 sec
Forward Delay = 15sec

Total Time = Max Age + (Forward Delay x Port States)

Scenario
*SW fails*

20 sec.

SW2 blocks and starts listening

15 seconds (listening... listening... listening)

SW2 stops listening and starts learning

15 seconds (learning... learning... learning)

SW2 is now learned and starts forward

Data Operation resumes.

Time Duration: 20 + 15 + 15 = 20 sec