Chapter 27 Loop Guard
GS2210 Series User’s Guide
258
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I t w ill r eceive br oadcast m essages sent out fr om the sw it ch in loop st at e.
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I t w ill r eceive it s ow n br oadcast m essages t hat it sends out as t hey loop back. I t w ill t hen r e-
br oadcast t hose m essages again.
The following figur e shows por t N on sw it ch A connect ed t o sw it ch B. Swit ch B is in loop st at e.
When br oadcast or m ult icast packet s leave por t N and r each sw it ch B, t hey ar e sent back t o por t N
on A as t hey ar e r ebr oadcast fr om B.
Figure 179
Sw it ch in Loop St at e
The loop guar d feat ur e checks t o see if a loop guard enabled por t is connect ed t o a sw it ch in loop
st at e. This is accom plished by per iodically sending a pr obe packet and seeing if t he packet r et ur ns
on t he sam e por t . I f t his is t he case, t he Swit ch w ill shut dow n t he por t connect ed t o t he sw it ch in
loop st at e.
The follow ing figur e show s a loop guar d enabled por t N on sw it ch A sending a pr obe packet P t o
sw it ch B. Since sw it ch B is in loop st at e, t he pr obe packet P r et ur ns t o por t N on A. The Sw it ch
t hen shut s dow n por t N t o ensure t hat t he r est of t he net w or k is not affect ed by t he sw it ch in loop
st at e.
Figure 180
Loop Guar d - Pr obe Packet
The Swit ch also shut s dow n por t N if t he pr obe packet r et ur ns t o sw it ch A on any ot her por t . I n
ot her w or ds loop guar d also pr ot ect s against st andar d net wor k loops. The following figur e
illust rat es t hr ee sw it ches form ing a loop. A sam ple pat h of t he loop guard pr obe packet is also
show n. I n t his ex am ple, t he probe packet is sent fr om port N and r et ur ns on anot her por t . As long
as loop guar d is enabled on por t N . The Sw it ch w ill shut dow n por t N if it det ect s t hat t he pr obe
packet has r et ur ned t o t he Swit ch.
A
B
N
A
B
P
P
N
