ZyXEL Communications XS1920 Series, User Manual

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Chapter 27 Loop Guard
XS1920 Series User’s Guide
254
• 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 173
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 174
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.
A
B
N
A
B
P
P
N