Figure 113 Switch In Loop State; Figure 114 Loop Guard - Probe Packet; Figure 115 Loop Guard - Network Loop - ZyXEL Communications ES-3148 Series User Manual

Chapter 25 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 113 Switch in Loop State

B
A
N
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 114 Loop Guard - Probe Packet

B
A
P
P
N
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 115 Loop Guard - Network Loop

A
N
P
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ES-3148 User's Guide