Chapter 28 Loop Guard
MES3500 Series User’s Guide
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Figure 137 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 138 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 139 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 8.6 on page 85) or via commands (see
the Ethernet Switch CLI Reference Guide).
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