Technical information

AX Series L2 Loop Detection Configuration Guide (Edition 2)

Impact of an L2 Loop and the Countermeasure Functionality

1.1 L2 loops and their risks

Networks are generally based on an assumption that the terminals and servers that send and receive data are

connected on an end-to-end basis both physically and logically. However, a data loop might be created due to

incorrect connections between devices used for network relaying (network devices) or incorrectly setting up

configurations on the network devices. If this occurs on a network controlled at the Layer 2 (L2) level, it is called an

L2 loop.

If an L2 loop is created, and then a broadcast frame enters, a serious problem occurs.

This causes an endless transfer of broadcast frames between the network devices and accordingly an increase in

the number of frames, leading to the compression of the transfer bandwidth that can be processed by the network

device and, in the worst case, results in a failure that stops other normal communication. (This condition is referred

to as a broadcast storm.)

Because broadcast frames are generally used for ARP requests and DHCP discovery in L2 networks, creating

such an L2 loop can cause a broadcast storm, which is very dangerous.

Figure 1.1-1 L2 loop and broadcast storm

[3] Congestion due to a storm

-> Normal communication sto

[2] Broadcasts continue to

flow and cause a storm!

[1] Incorrect connections cause a loop.