Clarification on Spanning-Tree (PortFast)

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Dec 4th, 2007
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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.


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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.



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




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