ZyXEL Communications XGS4700 Series, Manual

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Chapter 27 Loop Guard
XGS4700-48F User’s Guide
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• It will receive its own broadcast messages that it sends out as they loop back. It
will then re-broadcast those messages again.
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 135 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 136 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
A
B
N
A
B
P
P
N