Since the last year, the "KEK Secure Network System" completely replaced all network switches throughout the Tsukuba Campus of KEK. The new switch Apresia series from Hitachi-Cable enables the detection of loop connections. Because of careless attaching of the miniswitches to the existing wires, a network loop occurs about once a month (Fig. 4-4-2-2(a)). If the network loop is not correctly blocked, it can cause a meltdown of the entire KEK network. Before the switch replacement, the detection functionality was poor and the meltdown occurred a few times a year. When the switches were introduced, they detected the loop when both its ends were connected to an identical switch (Fig. 4-4-2-2(b)). During the summer power outage in 2009, the firmware was upgraded and can now detect loop that is connected to different switches (Fig. 4-4-2-2(c)). However, the loop has still not been eliminated completely, but the damage from the loop has been quarantined to the switch.
(from 2009 KEK annual report)