Loop Guard - ZyXEL Communications MSC1024G Series User Manual

Chapter 5 Access Control List Screens

5.19 Loop Guard

Configure loop guard to protect against loops on the edge of your network. Loop guard allows
you to configure the system to shut down a port if it detects that packets sent out on that port
loop back to the system. 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 69 Loop Guard vs. STP
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 system A connected to another switch B. Switch B has
mistakenly two ports, x and y, connected to each other. It forms a loop. When switch B
receives broadcast or multicast frames, they will be broadcast again to senders including port
N on system A.
Figure 70 Switch in Loop State
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Loop Guard
B
A
N
STP
x y
MSC1000G/1024G/1224G Series User's Guide