ZyXEL Communications ONU-2024 Series, User Manual

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Chapter 22 Loop Guard
ONU User’s Guide
170
22.1.2 What You Need to Know About the Loop Guard Screens
The following terms and concepts may help as you read through this chapter.
Network Loop Issues
Loop guard is designed to handle loop problems on the edge of your network. This can occur
when a port is connected to a ONU that is in a loop state. Loop state occurs as a result of
human error. It happens when two ports on a switch are connected with the same cable. When
a switch in loop state sends out broadcast messages the messages loop back to the switch and
are re-broadcast again and again causing a broadcast storm.
If a switch (not in loop state) connects to a switch in loop state, then it will be affected by the
switch in loop state in the following way:
• It will receive broadcast messages sent out from the switch in loop state.
• 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 88 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 ONU 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
ONU then shuts down port N to ensure that the rest of the network is not affected by the switch
in loop state.
A
B
N