Allied Telesis AlliedWare Plus Operating System Version 5.4.4C (x310-26FT,x310-26FP,x310-50FT,x310-50FP) - Manual (Page 355)

Switching Introduction
Software Reference for x310 Series Switches
C613-50046-01 REV A AlliedWare Plus TM Operating System - Version 5.4.4C 14.13
Loop Protection
Loop protection is a general term that embraces several different methods you can apply
to protect your network from effects such as broadcast storms that can result from data
loops or equipment malfunction.
Presently two methods of loop protection are available:
■ Loop Detection
■ Thrash Limiting
Loop Detection
Introduction
This feature is used to detect loops with a network segment. If a loop is detected then a
selected protection mechanism is applied to limit the effect of the loop. The loop
protection actions can be applied either to the port at which the loop is detected or to the
VLAN within which the loop was detected.
Limiting Actions You can configure loop detection to apply one of the following mechanisms when a loop
condition is detected:
■ Block all traffic on the port (or aggregated link) that detected the loop, and take down
the link.
■ Block all traffic on the port (or aggregated link) that detected the loop, but keep the
link in the up state.
■ Block all traffic on a vlan. Note that setting this parameter will also enable ingress
filtering. This is the default action.
■ Take no action, but log the details.
■ Take no action.
Operation
To detect loops this feature operates by transmitting a series of Loop Detection Frames
(LDFs) from each switch port out into the network. If no loops exist, then none of these
frame should ever return. If a frame returns to its original port, the detection mechanism
assumes that there is a loop somewhere in the network and offers a number of protective
options.
Each LDF is a Layer 2 LLC frame that contains the following components:
■ the source MAC address of the originating switch
■ the destination MAC address of the non-existent end station 00-00-F4-27-71-01
■ VLAN ID (where the port is a tagged member of a VLAN).
■ a randomly generated LDF ID number.
You can set the detection mechanism to remember the LDF ID of up to 5 of the most
recently transmitted LDF frames. Each of the 5 most recently transmitted frames is
compared with every frame that arrives at that same port.