Loop Guard - ZyXEL Communications GS3700 Series User Manual

This chapt er shows you how t o configure t he Swit ch t o guard against loops on t he edge of your
net work.
26.1 Loop Guard Overview
Loop guard allows you t o configure t he Swit ch t o shut down a port if it det ect s t hat packet s sent out
on t hat port loop back t o t he Swit ch. While you can use Spanning Tree Prot ocol ( STP) t o prevent
loops in t he core of your net work. STP cannot prevent loops t hat occur on t he edge of your
net work.
Figure 208 Loop Guard vs STP

Loop Guard

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.
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Loop Guard
STP