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Loop guard

I am planning to implement the enable the loop guard option. I Just want to Know will there be any STP topology change.( topology recalculation)

Cisco Employee

Re: Loop guard

No.. Configuring loop guard should not trigger any TCNs. But as always, it is best to make any changes in a maintenance window.


Re: Loop guard

Let me emphasize the thoughts of the previous post. When it comes to STP, NEVER, I mean NEVER, make ANY changes unless during scheduled maintenance hours.

Re: Loop guard

Unless you know what you are doing...

I have no problem configuring portfast on an access switch any time for instance. Changing the priority of the root bridge is of course a different issue;-) This kind of *NEVER* rule seems too extreme to me.

Loopguard should not make any change in the topology at all. If you can afford easily a maintenance window, wait for it.




Re: Loop guard

I suppose everybody thinks they know what they are doing when they are doing it.

Configuring portfast on a live switch can be very dangerous.


Two access switches are uplinked to a pair of core switches without portfast enabled on the access ports. A user has connected a hub in a conference room and mistakenly connected 2 ports on the hub to the network, one to each access switch. An engineer configures portfast on the access switches and a bridge loop is created. So much traffic floods through the loop that HSRP and routing protocols start breaking, STP is unable to break the loop. The whole network comes to a stand still.

This isnt something made up. This has actually happened. I was called to the scene to help clean up the mess.

Re: Loop guard

Unless because of a bug, which is an option never ruled out indeed:-(, configuring portfast does not put a blocked port into forwarding. There must have been a link change. The real issue was more the wrong cabling than the portfast config and this problem could have happened after your maintenance window. You must also ensure that cabling errors only occur during the maintenance window;-)

Anyway, I perfectly understand your point but I would not still not consider this as a strict rule written in capital letters.


Cisco Employee

Re: Loop guard

Well, isn't this a "USER ERROR" ?

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