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Cisco Loop Guard Confusion

Posted on 2010-09-24
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I have problems understanding loopguard. Firstly why would you put loop guard on a root port which is a forwarding state anyway??
Secondly when we put loop guard on what happens when when the neighboring switch legitmately goes down and the obviously the blocking port
stops recieving bpdu's from the switch that has gone down it then it goes into the loop inconsistent state rather than into a forwarding state for the segment does it not
take away the failover functionality of spanning tree requiring manual intervention??
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Question by:BarepAssets
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mikebernhardt earned 2000 total points
ID: 33756718
If the neighboring switch goes down, the link will go down with it (not just the bpdus) and loop guard won't interfere with normal spanning tree.
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by:jbrmiller
ID: 33757074
Loopguard doesn't use bpdus, so, it's not related to spanning tree.
Loopguard sends keepalives through all ports on the switch, and this frames are ignored by all neighborging switches, and are not forwarded. But, if a loop is created, and the switch receives his own keepalive in any port, the port is err-disabled.

the protocol that's related to spanning tree is bpduguard, and is intented for other purposes.
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by:BarepAssets
ID: 33759892

thaks for the replies

 mike that explains the link issue what about why do we put loop guard on root ports which are forwarding anyway

jbrmiller i think you may confusing loop guard with udld that is the protocol that is not related to spanning tree and sends keepalives loop guard is part of spanning tree tools and monitors bpdus as far as my study so far says anyway  
bpdu guard is for making sure if you mistakenly connect a switch to portfast enabled port it does not go straight away into forwarding state bypassing listening learning etc possible creating a loop it error disables on reciept of bpdu if enabled on interface
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by:mikebernhardt
mikebernhardt earned 2000 total points
ID: 33771635
The idea is to put it on all ports facing the root switch. The switch knows where the root is. If it starts receiving BPDUs from a port other than the one facing the root, it will assume that a loop has formed. In a failure scenario it's possible that the root port will no longer be the root port due to a spanning tree change even though the link is up. Now that port needs to have loop guard in case a loop isn't caught in time. So it's best practice to do it.

And let me tell you personal experience, a spanning tree loop on a modern high-speed network is a really bad thing.
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by:mikebernhardt
ID: 33771644
>The idea is to put it on all ports facing the root switch.

I mean, all ports with a potential path to the root switch, whether blocking or forwarding.
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by:BarepAssets
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