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PortFast

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salmans

IS-IT--Management
Aug 29, 2003
69
US
I need some clarification... I know that when you enable portfast on a switch interface and a device is connected to this interface it skips the STP blocking/listening/learning states and goes directly to forwarding state. And portfast is supposed to turn itself off on the interface if it receives a BPDU (meaning a switch had been plugged into that interface) and the interface should just run in normal STP mode. But what if you plug in another switch and it causes a Broadcast Storm/Loop, before portfast can turn itself off. After portfast does turn itself off, how long does it take for the broadcast storm/loop to clear?
In the switch it states that turning on PortFast can cause temporary loops and cisco's website says the same....my question is: How long do these temporary loops last? How long until they clear? Will it take 30 seconds? (i.e. after PortFast turns off and the interface goes back to normal it goes through the STP blocking/listening/learning/ phase (30 seconds'ish) and then the loop is cleared?
Does anyone have any idea? Thanks !!!!!!!

Do it right the first time, and there won't be a second time!
 
It is only going to cause a problem if you hook in that switch and it is looped between 2 ports on a existing switch , a single connected switch to a cisco switch is not going to cause any problems because their is no loop , if you loop it then there will be a loop probably for a long as takes for normal spanning tree to run 30-45 seconds and then it will put one port into a blocking state.
 
Use Portfast in conjunction with BPDU Guard and BPDU Filter and there shouldn't be a problem.

MCP,CCA, Net+, 1 quarter CCNP...
 
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