ZyXEL Communications MES-2110 CHAPTER 8

MES-2110 User’s Guide 71
CHAPTER 8
Loop Detection
8.1 Overview
Loop detection allows you to configure the MES-2110 to shut down a port if it
detects that frames sent out on that port loop back to the MES-2110.
Loop detection is designed to handle loop problems on the edge of your network.
This can occur when a port is connected to a MES-2110 that is in a loop state.
Loop state occurs as a result of human error. It happens when two ports on a
switch are connected with the same cable. When a switch in loop state sends out
broadcast messages the messages loop back to the switch and are re-broadcast
again and again causing a broadcast storm.
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 re-broadcast those messages again.
The following figure shows port N on switch A connected to switch B. Switch B is
in loop state. When broadcast or multicast frames leave port N and reach switch
B, they are sent back to port N on A as they are rebroadcast from B.
Figure 27 Switch in Loop State
The loop detection feature checks to see if a loop detection enabled port is
connected to a switch in loop state. This is accomplished by periodically sending a
probe frame and seeing if the frame returns on the same port. If this is the case,
the MES-2110 will shut down the port connected to the switch in loop state.
AB
N