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Confusion in time taken by STP to make a port in forwarding state

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Oct 24th, 2014
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Hello Guys,

I am little confused my doubt is that STP uses 50 sec or 30 sec to make a port in forwarding state. But i don't know in which case it will take 50 and in which case it wil take 30 please explain in detail.

Thanks in advance

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houtan haddadla... Sat, 10/25/2014 - 02:02
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Hi,

It directly depends on what STP feature you are enabled. 

for a STP enabled port, in normal state, it takes 20sec max age+ 15 sec listening + 15 sec learning to goes to forwarding state.

but if you are using Backbonefast feature it skips max-age timer and going to forwarding state takes 30sec.

 

HTH

Houtan

 

pankaj kumar Sat, 10/25/2014 - 06:33
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I was reading CISCO press CCNP switch book.

Consider the topology i have attached.

In this In normal condition without any feature if SW C 1/1 fails then SW C will take 30 sec to make its blocking port 1/2 in forwarding state.

But i think it should take 50 sec.

Attachment: 
houtan haddadla... Sat, 10/25/2014 - 06:46
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Ok, this is about direct link failure. in this state port directly goes to listening state because its line protocol goes to down-down state, therefore switch does not wait for max-age time.

 

My previous response was about indirect link failure (normal reconvergence time is 50sec+2sec receiving new BPDU) and the method for its fast convergence.

Houtan

Thiyagu VG Wed, 11/05/2014 - 00:41
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Please find few more updates about how fast an interface comes to forwarding state:

 

STP(pvst or iee):
-----------------------

Access Port without portfast/edgeport enabled. -- 30 sec (Initialize the port, listening to learning 15sec, learning to forwarding 15sec, total 30sec.)

Access Port with portfast/edgeport enabled. -- less than a sec (initialize stp on the port, then jump to forwarding)

Trunk Port where another Cisco Switch is connected which is also running STP. -- 30sec (listen to learn and learn to fwd)

Direct Link Failure without uplinkfast -- recovery is 30sec.(pkt drops)

Failed Direct Link restored without uplinkfast -- recovery is  30 sec. (pkt drops)

Direct Link Failure with uplinkfast -- recovery is  less than a msec. (no pkt drops)

Failed Direct Link restored with uplinkfast -- recovery is 35sec (2*Fwd Delay + 5 sec (to converge other protocol like etherchannel, etc..)) with no pkt 

drop

as soon as the actual (previous) link comes up, it goes to listen, then after 300 msec, it goes to root port delay timer and blocking state. Then after 35 

sec... it goes to Root Port (Forwarding state).


Indirect link failure without backbonefast -- recovery is around 47 sec. (17-20 sec for max-age timer, 2*fwd_delay listen, learn, fwd.)

MM:SS:Msec

09:10:002 -- Heard Root
09:27:414 -- Listen
09:42:413 -- Learn
09:57:413 -- Forward


Indirectl link failure with backbonefast -- recovery is 30sec. (2*fwd_delay listen, learn, fwd.). Excluding max-age timer, instead sends and rcv RLQ 

within 16 msec.

31:35:320 - Heard Root
31:35:346 - Listen
31:50:346 - Learn
32:05:346 - Forward

--------------------------------------------------------------------------------------------------------------------

802.1w (Rapid PVST+):
-------------------------------

Access Port without portfast/edgeport enabled. -- 30 sec. (Initialize the port, sends (18) proposal and if no reply, then the port will be up)

Access Port with portfast/edgeport enabled. -- less than a sec (initialize stp on the port, then goes to designated)

Trunk Port where another Cisco Switch is connected which is also running rapid-pvst+ -- 2.7 sec...

Trunk Port where another Cisco Switch is connected which is running pvst --- 30 sec.(Time is high due to absense of proposal/aggrement)

Direct Link Failure -- recovery is less than a sec. - with no pkt drops.

Failed Direct Link Restored -- recovery is  than 2 sec with no pkt drops.

Indirect Link failure.. -- recovery is less than a sec..with no pkt drops.

Indirect Link failure recovered..... -- recovery is less than a sec..with no pkt drops.

-----------------------------------------------------------------------------------------------------------

802.1s (MSTP):
--------------------

Access Port without portfast/edgeport enabled. -- 30 sec. (Initialize the port, sends (18) proposal and if no reply, then the port will be up)

Access Port with portfast/edgeport enabled. -- less than a sec (initialize stp on the port, then goes to designated)

Trunk Port where another Cisco Switch is connected which is also running MSTP. -- 2.7 sec...

Trunk Port where another Cisco Switch is connected which is running rpvst+ -- 3.5 sec...

Trunk Port where another Cisco Switch is connected which is running pvst --- 30 sec.(Time is high due to absense of proposal/aggrement)


Direct Link Failure -- recovery is less than a sec. - with no pkt drops.

Failed Direct Link Restored -- recovery is  than 2 sec with no pkt drops.

Indirect Link failure.. -- recovery is less than a sec..with no pkt drops.

Indirect Link failure recovered..... -- recovery is less than a sec..with no pkt drops.

---------------------------------------------------------------------------------------------------------------------

 

Thanks,

Thiyagu Ganesan.

jfloresr Sun, 10/26/2014 - 06:32
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  • Cisco Employee,

Hi,

Like Houtan mentioned above it depends on the features you have configured but I think your question is more towards failure scenarios.

It depends on how "graceful" the failure is. If there is an unidirectional link failure or a cable cut failure the switch will wait its normal 30 sec hold + lis + learn but if you shutdown the port, the switch being a little bit "smart" about it will detect the shutdown and skip the hold timer.

HTH

"Please rate the post if helpful."

Joseph W. Doherty Wed, 11/05/2014 - 05:43
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You might also want to read: http://blog.ine.com/wp-content/uploads/2011/11/understanding-stp-rstp-convergence.pdf

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