ZyXEL ES-4124 Manual Do Utilizador

Chapter 25 Loop Guard

ES-4124 User’s Guide

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 113   Switch in Loop State

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. 

Figure 114   Loop Guard - Probe Packet

The Switch also shuts down port N if the probe packet returns to switch A on any other port. In 
other words loop guard also protects against standard network loops. The following figure 
illustrates three switches forming a loop. A sample path of the loop guard probe packet is also 
shown. In this example, the probe packet is sent from port N and returns on another port. As 
long as loop guard is enabled on port N. The Switch will shut down port N if it detects that the 
probe packet has returned to the Switch. 

Figure 115   Loop Guard - Network Loop