ZyXEL Communications MSC1000G User Manual page 124

Chapter 4 Access Control List Screens
Figure 59 Switch in Loop State
x y
B
A
N
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 60 Loop Guard - Probe Packet
B
A
N
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.
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Management Switch Card User's Guide