Loop Guard; Chapter 28 Loop Guard; Loop Guard Overview - ZyXEL Communications MES3500 series User Manual

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Loop Guard

This chapter shows you how to configure the Switch to guard against loops on the edge of your
network.

28.1 Loop Guard Overview

Loop guard allows you to configure the Switch to shut down a port if it detects that packets sent out
on that port loop back to the Switch. While you can use Spanning Tree Protocol (STP) to prevent
loops in the core of your network. STP cannot prevent loops that occur on the edge of your
network.
Figure 136 Loop Guard vs STP
STP
Loop Guard
Loop guard is designed to handle loop problems on the edge of your network. This can occur when
a port is connected to a Switch that is in a loop state. Loop state occurs as a result of human error.
It happens when two ports on a switch are connected with the same cable. When a switch in loop
state sends out broadcast messages the messages loop back to the switch and are re-broadcast
again and again causing a broadcast storm.
If a switch (not in loop state) connects to a switch in loop state, then it will be affected by the
switch in loop state in the following way:
• It will receive broadcast messages sent out from the switch in loop state.
• It will receive its own broadcast messages that it sends out as they loop back. It will then re-
broadcast those messages again.
The following figure shows port N on switch A connected to switch B. Switch B is in loop state.
When broadcast or multicast packets leave port N and reach switch B, they are sent back to port N
on A as they are rebroadcast from B.
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