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Loop Guard

Posted on 2015-02-24
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An article about Loop Guard and I want to clarify my understanding

Loop guards : The switch figures out which ports are nondesignated and monitors the BPDU activity to keep them nodesignated. Nodesignated ports are generally the root port, alternate root ports and ports that normally are blocking.

BDPU is forward by Designated Port and received by Root Port on another switch.  Why does it say root port is blocking ?

Do I misunderstand something ? Tks
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Question by:AXISHK
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by:Don Johnston
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The switch figures out which ports are nondesignated and monitors the BPDU activity to keep them nodesignated.
This is correct.
Nodesignated ports are generally the root port, alternate root ports and ports that normally are blocking.
No.  nondesignated ports are any ports which received inferior BPDU's (BPDU's that have a higher cost than the preferred BPDU). "Blocking" is another way of saying "nondesignated".
BDPU is forward by Designated Port and received by Root Port on another switch.
This is correct.

But none of what you've said is really related to loopguard.  So I'm not sure what it is you're misunderstanding.
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by:AXISHK
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The original message is
" You can enable loop guard on all switch ports, regardless of their functions. The switch figures out which ports are nondesignated and monitors the BPDU activity to keep them nodesignated. Nodesignated ports are generally the root port, alternate root ports and ports that normally are blocking."

Nodesignated ports are generally the root port, alternate root ports and ports that normally are blocking.

Is Root port blocked ? Base on my understanding, "Block" means that the port can receive BPDU but can't forward it out, or other frame, correct ? Tks
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by:Don Johnston
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Can you post a link to the article?  

Is Root port blocked ?
No. Root ports are never blocked. If they were, they wouldn't be root ports.

Base on my understanding, "Block" means that the port can receive BPDU but can't forward it out, or other frame, correct ?
This is correct.
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by:AXISHK
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by:Don Johnston
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Yeah, that's the standard Cisco document on loop guard. Sometimes I think they write that stuff to intentionally confuse people.
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by:AXISHK
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just to clarify

Root port can only receive BPDU but it can't forward BPDU and frame. In fact the port does not block but Cisco just interrupt it as "blocked" in a the sense that root port doesn't forward any packet, correct ?

Tks again.
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by:Don Johnston
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No. The root port receives and processes BPDU's, and data frames.  It also sends data frames.

A root port is classified as a forwarding port.
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by:AXISHK
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Tks, so root port receives and processes BPDU but it do not forward BPDU, BDPU should be forward out by Designated port, correct ?
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Don Johnston earned 500 total points
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Correct.
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