Monitor: Loop Guard; Chapter 12 Monitor: Loop Guard; Overview - ZyXEL Communications GS1900 Series User Manual

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Monitor: Loop Guard

12.1 Overview

This section provides information for Loop Guard in Monitor.
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.
Figure 66 Switch in Loop State
B
A
N
The loop guard feature checks to see if a loop guard enabled port is connected to a switch in loop
state. This is accomplished by periodically sending a probe packet and seeing if the packet returns
on the same port. If this is the case, the Switch will shut down the port connected to the switch in
loop state.
The following figure shows a loop guard enabled port N on switch A sending a probe packet P to
switch B. Since switch B is in loop state, the probe packet P returns to port N on A. The Switch
then shuts down port N to ensure that the rest of the network is not affected by the switch in loop
state.
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