ZyXEL Communications VES1724-56 User Manual page 271

Chapter 27 Loop Guard
broadcast or multicast frames, they will be broadcasted again to senders including the port N on
the Switch A.
Figure 162 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 Switch 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 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 163 Loop Guard - Probe Packet
B
A
N
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 the Switch 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
271
VES1724-56 User's Guide