ZyXEL ES-3148 Guida Utente

ES-3148 User’s Guide

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

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 112   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.