Chapter 22 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 89 Switch in Loop State
AB
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 90 Loop Guard - Probe Packet
AB
P 
N
P
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 91 Loop Guard - Network Loop
A
P
N
P 
P
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