Loop Guard Setup - ZyXEL Communications OLT2406 User Manual

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 148 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 OLT 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 OLT then shuts down port
N to ensure that the rest of the network is not affected by the switch in loop state.
Figure 149 Loop Guard - Probe Packet
The OLT 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
OLT will shut down port N if it detects that the probe packet has returned to the OLT.
Figure 150 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

28.2 Loop Guard Setup

Click Advanced Application > Loop Guard in the navigation panel to display the screen as shown.
Note: The loop guard feature can not be enabled on the ports that have Spanning Tree
Protocol (RSTP or MSTP) enabled.
Chapter 28 Loop Guard
Section 8.7 on page
OLT2406 User's Guide
247
104).