Chapter 49 Switching
GS1350 Series User’s Guide
349
Figure 255
Loop Guard vs. STP
49.6.1 What You Need to Know
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:
• The switch (not in loop state) will receive broadcast messages sent out from the switch in loop state.
• The switch (not in loop state) 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
has two ports,
x
and
y
,
mistakenly connected to each other. It forms a loop. 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 256
Switch in Loop State
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.
Loop guard can be enabled on both Ethernet ports. 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.
