ZyXEL Communications MSC1024G Series User Manual page 133

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 system will shut down the port
connected to the switch in loop state.
Loop guard can be enabled on both Ethernet ports or xDSL ports. In the following figure,
Ethernet port N has loop guard enabled on the system 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 system then
shuts down port N to ensure that the rest of the network is not affected by the switch in loop
state.
Figure 71 Loop Guard - Probe Packet
The system also shuts down port N if the probe packet returns to system A on any other port.
In other words loop guard also protects against standard network loops.
The following figure illustrates the system A, a subscriber device B and another switch C
forming a loop. A sample path of the loop guard probe packet is also shown. In this example,
the probe packet is sent from an xDSL port 1 and returns also on port 1. As long as loop guard
is enabled on port 1, the system will shut down port 1 if it detects that the probe packet has
returned to the system.
Figure 72 Loop Guard - Network Loop
After resolving the loop problem on your network you can re-activate the
disabled port via the Web Configurator or via commands.
MSC1000G/1024G/1224G Series User's Guide
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Chapter 5 Access Control List Screens
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