Monitor: Loop Guard - ZyXEL Communications GS1900-8 User Manual

12.1 Overview
This sect ion provides inform at ion for Loop Gua r d in M onit or.
Loop guard is designed t o handle loop problem s on t he edge of your net work. This can occur when
a port is connect ed t o a Swit ch t hat is in a loop st at e. Loop st at e occurs as a result of hum an error.
I t happens when t wo port s on a swit ch are connect ed wit h t he sam e cable. When a swit ch in loop
st at e sends out broadcast m essages t he m essages loop back t o t he swit ch and are re- broadcast
again and again causing a broadcast st orm .
I f a swit ch ( not in loop st at e) connect s t o a swit ch in loop st at e, t hen it will be affect ed by t he
swit ch in loop st at e in t he following way:
• I t will receive broadcast m essages sent out from t h e swit ch in loop st at e.
• I t will receive it s own broadcast m essages t hat it sends out as t hey loop back. I t will t hen re-
broadcast t hose m essages again.
The following figure shows port N on swit ch A connect ed t o swit ch B. Swit ch B is in loop st at e.
When broadcast or m ult icast packet s leave port N and reach swit ch B, t hey are sent back t o port N
on A as t hey are rebroadcast from B.
Figure 66 Swit ch in Loop St at e
The loop guard feat ure checks t o see if a loop guard enabled port is connect ed t o a swit ch in loop
st at e. This is accom plished by periodically sending a probe packet and seeing if t he packet ret urns
on t he sam e port . I f t his is t he case, t he Swit ch will shut down t he port connect ed t o t he swit ch in
loop st at e.
The following figure shows a loop guard enabled port N on swit ch A sending a probe packet P t o
swit ch B. Since swit ch B is in loop st at e, t he probe packet P ret urns t o port N on A. The Swit ch
t hen shut s down port N t o ensure t hat t he rest of t he net work is not affect ed by t he swit ch in loop
st at e.
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GS1900 Series User's Guide
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Monitor: Loop Guard

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