ZyXEL Communications XGS1930-38 User Manual page 208

Chapter 26 Loop Guard
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 143 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 144 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 145 Loop Guard - Network Loop
Note: After resolving the loop problem on your network you can re-activate the disabled port
via the web configurator (see Section 8.7 on page 83).
XGS1930 Series User's Guide
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