Loop Prevention; Examples With Multiple Loop Prevention Switches - Allied Telesis GS910 Series Installation And User Manual

Loop Prevention

Examples with
Multiple Loop
Prevention
Switches
The GS910 series switches are equipped with Loop Prevention, a feature
that detects loops and blocks ports in order to reduce negative effects on
the local network while keeping connectivity of devices. Loops in Ethernet
networks can cause broadcast storms that consume network bandwidth
and reduce network performance.
When Loop Prevention is enabled, the switch sends Loop Prevention
frames periodically and detects a loop in the LAN when a port receives the
Loop Prevention frame sent by the port itself. Then, the switch applies the
loop prevention algorithm to block ports to relieve the loop.
When the switch detects a loop and blocks a port, the following actions are
taken:
The Loop LED starts blinking.

The LED of the blocked port starts blinking.

In a topology with multiple GS910 series switches, these switches elect a
root switch when Loop Prevention is enabled on all the switches. Initially,
these switches are all root switches to send Loop Prevention frames,
compare their MAC address, and elect a switch with the largest MAC
address as a root switch. After a root switch is elected, the root switch
sends Loop Prevention frames from its ports every two seconds. A
non-root switch updates the Loop Prevention frames and forwards them. A
non-root switch also maintains a timer for each port and sets a timer to 16
seconds. When receiving a Loop Prevention frame at a port, a non-root
switch refreshes the timer of the port. When all timers of the non-root
switch reach zero, the non-root switch changes itself to a root switch and
sends Loop Prevention frames.
In examples shown in Figure 13 on page 24, multiple GS910 series
switches form links and all switches are Loop Prevention enabled. Switch
C has a larger MAC address than Switch A and Switch B; Switch B has a
larger MAC address than Switch A. First, these switches elect a root
switch by comparing the MAC addresses. In cases 1 and 2, Switch B is
elected as a root switch; in case 3, Switch C is elected as a root switch.
Then the root switch initiates Loop Prevention frames and non-root
switches update the frames and forward them. When detecting a loop, the
root switch runs the Loop Prevention algorithm to decide which port to
block, and blocks a port or ports to relieve the loop.
Chapter 1: Product Description
23