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New Member

Spanning Tree and Portfast

Hi,

I'm wondering what would happen if you connect two switches together with two up-links where all 4 ports are configured as access ports with portfast enabled? No BPDU Guard.

I know all ports should transition into forwarding state immediately and create a loop but will one port get disabled after a period of time? I understand portfast doesn't actually disable STP.

I can't simulate this in Packet Tracer and GNS3 is not very switch friendly :)

Thanks for any help.

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Hall of Fame Super Silver

The difference is quite

The difference is quite simple. If you make those connections with portfast enabled there will be a (short) burst of activity as the loop forms and then is detected and the loop is broken. If you make those connections with portfast disabled then spanning tree will do its usual listening, learning, forwarding progression in which the loop will be detected (and prevented) and there is no burst of activity.

 

HTH

 

Rick

5 REPLIES

HelloBy default spanning-tree

Hello

By default spanning-tree is enabled on the switches, So stp will block one the the ports to negate a loop - however if you disable stp then a loop will occur and your switch cpu utilization will hit the roof and prohibit communication

res

Paul

Please don't forget to rate any posts that have been helpful. Thanks.
Hall of Fame Super Silver

I have done this on real

I have done this on real switches and can say that after a burst of activity that spanning tree did detect the loop and did put one port into the blocking state.

 

HTH

 

Rick

New Member

Sorry, forgot to mention that

Sorry, forgot to mention that STP is enabled.

So there would be a loop for a period of time until STP figures it out and disables one port? And if the more switches in the network the long it would take to settle down?

Basically, I'm trying to figure out the difference if you did the same thing with portfast enabled and disabled.

Hall of Fame Super Silver

The difference is quite

The difference is quite simple. If you make those connections with portfast enabled there will be a (short) burst of activity as the loop forms and then is detected and the loop is broken. If you make those connections with portfast disabled then spanning tree will do its usual listening, learning, forwarding progression in which the loop will be detected (and prevented) and there is no burst of activity.

 

HTH

 

Rick

New Member

Cool, that's what I wanted to

Cool, that's what I wanted to know.

Had a weird issue where there was a lot of '%SW_MATM-4-MACFLAP_NOTIF' events logged for all the switches for about 5min, caused a short outage (loop like symptoms) and then everything went back to normal. Pretty much at the same time as the FLAP events stopped there was an event on one of the switches saying a port was disabled because of an STP loop.

I'm just surprised it took so long.

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