spanning tree

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Mar 5th, 2008
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Hi all, can anyone tell me why we have spanning tree per vlan ? and also i have heard that the most switches you can have daisy chained is 7 due to the timers, what timers are these ?

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h2odata_ab Wed, 03/05/2008 - 10:01
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I don't know why you have spanning tree par vlan.

Regarding your second question I haven't heard a specific number like 7 before for number of daisy chained switches. The reason you can't daisy chain an unlimited number of switches is latency. Every switch hop means some delay for a packet in transfer. If you have too many switches daisy chained the packet will timeout.

mike15400 Wed, 03/05/2008 - 11:00
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It is not true that the "frames" will "timeout".

The daisy chain limit is to ensure that the Spanning Tree will converge in a loop free topology. All of the recommended timers are based on a maximum network "diameter" of 7.

craig.eyre Wed, 03/05/2008 - 11:34
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PVST maintains a spanning tree instance for each VLAN configured in the network. It uses ISL Trunking and allows a VLAN trunk to be forwarding for some VLANs while blocking for other VLANs (not the case for normal STP). Since PVST treats each VLAN as a separate network, it has the ability to load balance traffic (at layer-2) by forwarding some VLANs on one trunk and other Vlans on another trunk without causing a Spanning Tree loop.

Hope this helps.



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