Clarification on Spanning-Tree (PortFast)

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Dec 4th, 2007
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Hi,


I'm asking this question so i can get a good understanding of the matter.


If i connect a hub (uplink) to a 2950 switch and this port has portfast enabled, we'll i create a switching loop?

or do i need to connect a second cable from my hub to my 2950 switch.


Which scenario will create a switching loop. Or will both create a swithing loop.


thx


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Jon Marshall Tue, 12/04/2007 - 12:58
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Hi Tony


If you only have one connection from the hub then you should not create an STP loop. This is rather a wide statement because it's assuming you don't have anything else hanging off the hub that then connects back to the switch but in general no you won't get a switching loop.


With 2 you will get a switching loop assuming that the 2 connections from the hub go into ports that are in the same vlan. If the ports are in different vlans and your are running PVST+ (Per vlan spanning Tree) you won't.


HTH


Jon

royalblues Tue, 12/04/2007 - 13:20
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In the first case it will not create a loop,


In the second case if you are connecting another cable to a port on the switch having portfast configured, the new port will go into forwareding mose immediately and thus would create a transient loop.


This loop will stop as soon as BPDU's are heard from the respective ports.


But if the looped traffic is excessive, then it may bring down the network


HTH

Narayan

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