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STP and Timers

Posted on 2011-02-12
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Last Modified: 2012-05-11
any expert out there to explain the relation between STP and the timers used in STP:

for example:

Hello Time
Max Age
Forward Delay
-------------
Blocking
Listening
Learning
Forwarding

Thanks

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Question by:jskfan
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surbabu140977 earned 167 total points
ID: 34878978
hello time is 2 seconds
The Max Age Timer is 10x the the hello timer.
STP has 4 states: blocking, listening, learning and then forwarding
Once a port is in blocking state, it stays there for 20 seconds. Then moves onto listening at 15 seconds, then learning at 15 seconds. That's where you get your 50 seconds

RSTP
------
The max age is 3x the hello. So a max of 6 seconds. There's no blocking port in RSTP. It's discarding state. Discarding replaces blocking and listening. So you only have discarding, learning and forwarding.

forward_delay = ((4 x hello) + (3 x dia)) / 2
the Forward Delay Timer specifies the time a port stays in each of the listening and learning states

Example from cisco site,

hello = 2 (default) then
max_age = 14 sec
forward_delay = 10 sec
If hello = 1 then
max_age = 10 sec
forward_delay = 8 sec

Best,
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Expert Comment

by:Don Johnston
ID: 34879557
Can you elaborate as to what it is that you want to know?
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Author Comment

by:jskfan
ID: 34882486
I wanted to know how they are related, for instance if there is change in Hello time, Max age and Forward Delay, how it is going to impact the blocking ,listening,learning, forwarding operations
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by:SeeMeShakinMyHead
SeeMeShakinMyHead earned 167 total points
ID: 34882569
I would read this article as it explains it all.  If you are using spanning-tree instead of routed links, then I would use RSTP as it is much faster.  There are some sites where I have to use STP and I use RSTP since recovery on failed links it quite a bit faster.  So fast, that users really couldn't tell.

http://www.cisco.com/en/US/tech/tk389/tk621/technologies_tech_note09186a0080094954.shtml
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Author Comment

by:jskfan
ID: 34889795
After reading, I noticed that Hello,Max age, Forward Delay are pretty much other names used for blocking, listening, learning , forwarding.

Hello (02)+ Max Age (20) =  they can be mapped to Blocking state
Forward Delay= Can be mapped to Listening(15) + Learning(15)

the only difference is on the switch there are commands that can change  the Hello, Max age, Forward delay timers
but no command that can change  Blocking ,listening, learning, forwarding
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by:Don Johnston
Don Johnston earned 166 total points
ID: 34889899
Blocking, listening and learning are port states that are the RESULT of timer values.

If, for example, you wanted a failure to be detected quicker, you could lower the Max Age timer. And if you wanted to reduce the amount of time spent in listening/learning, you could reduce the Forward Delay timer. But tweaking timers can be problematic if you aren't careful.

For example, in almost 20 years of working with Spanning Tree, I have never altered the timers in a production network.
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by:jskfan
ID: 34890136
thank you guys!
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