James Koon and his IT department were reorganizing server racks to make more space in the server room. No big...
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Koon ran a Cat-5 wire to an electrical jack and a wire from the corresponding patch panel port to the switch. He booted up the server, but for some reason the server would not register a network connection.
"Being the troubleshooting IS guy I am, I grabbed a Cat-5 Wire Tester and plugged in the loopback where the Cat-5 plugged into the server," Koon said.
Then he took the other end of the wire and plugged it into the testing module. No reading came up. So he checked the connections and found something loose in the wire jack. He fixed the connection, unplugged the module tester, and plugged the module tester into the switch.
"Well, me and a few buddies started talking about some thing or another," Koon said. "Suddenly someone came into the computer room and said users were complaining they couldn't access e-mail or our Unix server or log onto the domain."
The network was down, and users couldn't even get an Internet connection.
Koon again checked the switch for a partially disconnected power supply, a loose wire, something. He couldn't find anything wrong.
A colleague logged onto a workstation, and he could not "ping" anything. While his colleague was in "mid-ping," Koon unplugged the cable from the switch. The pings began to make contact. He plugged the cable back into the switch, and the pings failed.
"I scratched my head," Koon said. "Then it dawned on me."
The loopback wire tester was still plugged in.
Koon unplugged the loopback module, and the network connections functioned.
The moral, said Koon, is simple: "Never plug a loopback device into a switch unless you want to tick off your users and helpdesk."