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Portfast enabled port went to blocking

Answered Question
Jun 9th, 2006
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One of my sysadmins created a loop on one of my 2948s. He daisy chained the ethernet console ports of a bunch of suns and patched the top and bottom machine to the 2948. The ports on the 2948 were all configured to portfast enabled. Yet somehow the switch still figured out there was a loop and put one of the ports into spanning tree blocked state. How did the switch succeed in blocking one port depsite there being portfast enabled? Thanks.

Correct Answer by Richard Burts about 11 years 2 months ago

The original post seems to have assumed that postfast would disable spanning tree (which is a very common - but incorrect assumption). The two responses were quite correct in explaining that portfast does not disable spanning tree but only puts the port immediately into forwarding mode. The port continues to process BPDUs and to participate in the spanning tree. Since BPDUs are sent every two seconds there will be a check for possible loop every two seconds.


HTH


Rick

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henniae Fri, 06/09/2006 - 16:14
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I don't think that the portfast command prevents the switch from finding spanning tree loops. My understanding is that portfast tells the switch not to do the standard "blocking, listening, learning, forwarding" tests when the switch port goes from down to up, it just goes right into forwarding mode. This speeds up port activation. If a loop is found after the port is up the switch still needs to block otherwise you have a meltdown.

Roberto Salazar Fri, 06/09/2006 - 18:00
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Spanning-tree PortFast causes a switch or trunk port to enter the spanning-tree forwarding state immediately, bypassing the listening and learning states.


You can use PortFast on switch or trunk ports that are connected to a single workstation, switch, or server to allow those devices to connect to the network immediately, instead of waiting for the port to transition from the listening and learning states to the forwarding state.

Caution You can use PortFast to connect a single end station or a switch port to a switch port. If you enable PortFast on a port that is connected to another Layer 2 device, such as a switch, you might create network loops.


When the switch powers up, or when a device is connected to a port, the port enters the spanning-tree listening state. When the Forward Delay timer expires, the port enters the learning state. When the Forward Delay timer expires a second time, the port transitions to the forwarding or blocking state.


When you enable PortFast on a switch or trunk port, the port transitions immediately to the spanning-tree forwarding state.



There is no mention that STP is disabled at all. It will immediately go to forwarding this might cause a loop but there is not mention that STP will not kick in at all.

mmedwid Fri, 06/09/2006 - 18:36
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So after the two ports go automatically into forwrding state from the portfast enabling - what happens that the switch figures out that a loop has been created? Is there a recurring listening/learning/forward/block process after the initial forced forwarding? How often does that occur?

Correct Answer
Richard Burts Sun, 06/11/2006 - 12:42
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The original post seems to have assumed that postfast would disable spanning tree (which is a very common - but incorrect assumption). The two responses were quite correct in explaining that portfast does not disable spanning tree but only puts the port immediately into forwarding mode. The port continues to process BPDUs and to participate in the spanning tree. Since BPDUs are sent every two seconds there will be a check for possible loop every two seconds.


HTH


Rick

mmedwid Sun, 06/11/2006 - 16:34
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Great - thank-you. What I was trying to guage is what the risk is in enabling portfast. If BPDUs continue to be sent every couple of seconds - it would seem pretty minimal in most environments.

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