electing root bridge

Unanswered Question
Jul 14th, 2008

i am confused abaut sth.Do we have to wait 20 seconds to elect the root bridge while switches are running 802.1d ? can it be faster ? is always the convergence time 20+15+15=50 seconds ?

I have this problem too.
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lee.reade Mon, 07/14/2008 - 04:21


There are a number of timer based steps in spanning-tree with 802.1d, basically the 20 second timer is for max-age, which is like a hold time for a specific port, if a switch receives bpdu's (spanning-tree packets) from a port, then that packet is valid for 20 seconds, if none are received within 20 seconds, then the switch starts to go thru convergence to select a new port.

You can tune the timers for 802.1d, but this can have an adverse effect on the network due to constant topology changes etc, if you are looking for fast convergence then you should look at switching to 802.1w, rapid spanning-tree.




soztekin326758 Mon, 07/14/2008 - 04:37

i mean without tuning timers,is it always 50 seconds ?

Thank you for your answer.

lee.reade Mon, 07/14/2008 - 04:44


By default yes, you have 50 seconds for STP convergence, however sometimes only some of the timers take effect.

It really depends if you deploy advanced features such as uplinkfast,backbonefast etc.

These features reduce the convergence times, when certain events happen.



Francois Tallet Mon, 07/14/2008 - 19:54

The 50 seconds can be broken down in:

- 20 second to age out some information. This generally does not happen in a modern network with point to point links. When a link goes down, each end detects the failure and the associated information is aged out immediately.

- 30 seconds to move a port from blocking to forwarding. This is with regular STP. Portfast and uplinkfast were already enhancing the convergence for most of the case. With RSTP and MST, in a correctly configured network with p2p links, you should be able to move a port from blocking to fowarding with no timer intervention.

So the 50 seconds that is still advertised everywhere is really a worse case scenario, most relevant to an older version of the standard (STP has been deprecated by RSTP: 802.1D *is* RSTP now!)




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