Chapter 26 Loop Guard
connected with the same cable. When a switch in loop state sends out broadcast messages the messages loop back to the switch and are
If a switch (not in loop state) connects to a switch in loop state, then it will be affected by the switch in loop state in the following way:
•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
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 130 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 131 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
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