spanning tree timers

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Sep 9th, 2008
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hi all, I have heard in the past about spanning tree diameter and the 1 in 7 rule for daisy chaining switches causes spanning tree timer issues, can anyone tell me what this is?

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satish_zanjurne Tue, 09/09/2008 - 05:28
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Hi,


diameter of the STP domain (dia)-This value is the maximum number of bridges between any two points of attachment of end stations. The IEEE recommendation is to consider a maximum diameter of seven bridges for the default STP timers.


Use the diameter keyword to specify the Layer 2 network diameter (the maximum number of bridge hops between any two end stations in the network). When you specify the network diameter, a switch automatically picks an optimal hello time, forward delay time, and maximum age time for a network of that diameter. This can significantly reduce the spanning tree convergence time.


You can use the hello-time keyword to override the automatically calculated hello time


But , it is recommended that you avoid manually configuring the hello time, forward delay time, and maximum age time after configuring the switch as the root bridge


HTH..rate if helpful..

Francois Tallet Tue, 09/09/2008 - 14:56
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Check out:

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


The "fear" associated with STP diameter is generally not founded;-) You should not worry too much about this, unless you are building rings.

In the meantime, I would recommend you use the "spanning-tree root" macro, which has a diameter option, in order to tune your timers if you really want to.

Regards,

Francois

carl_townshend Wed, 09/10/2008 - 03:11
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thanks, can you tell me the diff between stp and rstp convergence times? how come rstp is quicker, is it because it already has the alernate port ready to become active and des not need to recalculate costs etc ?

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