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spanning tree

carl_townshend
Spotlight
Spotlight

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 ?

3 Replies 3

h2odata_ab
Level 1
Level 1

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.

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
Level 1
Level 1

Hi,

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.

Craig

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