ZyXEL Communications Plumbing Product metrogigabit switch User Manual

MGS3700-12C 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 142   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.