Monitor: Loop Guard; C Ha Pte R 12 Mo Nito R: L O O P G Ua Rd; Overview - ZyXEL Communications GS1900 Series User Manual

12.1 O ve rvie w
This section provides information for
Loop guard is designed to handle loop problems on the edge of your network. This can occur when a
port is connected to a Switch 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
broadcast or multicast packets leave port
they are rebroadcast from
Switch in Loop State
Fig ure 87
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
. Since switch
B
port to ensure that the rest of the network is not affected by the switch in loop state.
N
Mo nito r: L o o p G ua rd
L o o p G ua rd
on switch
N
.
B
is in loop state, the probe packet
B
GS1900 Series User's Guide
C
HA PTER
in .
Mo nito r
connected to switch
A B
and reach switch , they are sent back to port
N B
on switch
N A
returns to port
P
87
. Switch is in loop state. When
B
sending a probe packet
on . The Switch then shuts down
N A
1 2
on as
N A
to switch
P