Neal Spangler

Work Security & network

Three long-running network faults found and fixed

The network had three unexplained faults: a switch that couldn't be managed, Wi-Fi that dropped every minute or two, and a Mac's wired port written off as broken. I traced each to its real cause and fixed it.

Managed network switch
Managed network switch Photo: Антон Пищулин, CC BY-SA 4.0
~7 monthsa hung switch went unnoticed
~4 monthsa working Mac port was blamed on hardware
0Wi-Fi drops captured after the fix

The problem

The switch still passed traffic while its management interface was hung. Wi-Fi dropped despite a good signal. The Mac's wired port had a link and an address, but direct traffic failed, which made it look like a hardware fault.

What I did

  • Recovered a switch whose management side had hung while it kept forwarding traffic. Backed up every switch config and upgraded firmware overnight, the second switch only after the first came back healthy.
  • Proved the Wi-Fi drops came from one network name served by a WPA3-only access point and a WPA2-only one. A macOS disconnect code pointed to it. Matching their security settings fixed it.
  • Found that the Mac's wired interface stopped replying while Wi-Fi was active on the same subnet. Kept one interface active and added a fleet check.
Three network faults: symptom, root cause, fix Row one shows a switch that was unreachable while traffic still flowed. Its management side had hung. It was recovered and upgraded overnight with an automatic abort. Row two shows Wi-Fi dropping despite good signal. One network name was served by a WPA3-only and a WPA2-only access point. Matching the security settings fixed it. Row three shows a Mac with link but no traffic on its wired port. Wi-Fi and Ethernet shared a subnet, and direct replies failed on the wired interface. The Mac was kept on one interface, and a fleet-wide check was added. Symptom Root cause Fix Switch unreachable traffic still flowing Hung management forwarding kept working Recover, upgrade overnight, auto-abort Wi-Fi drops despite good signal One name, two APs WPA3-only + WPA2-only Match security same mode on both Dead Mac port link up, no traffic Shared subnet macOS dual-homing One interface plus a fleet-wide check
The symptoms, causes and fixes for the switch, Wi-Fi and wired Mac connection.

What I took from it

Broadcast traffic still worked on the Mac's wired port, so the early checks sent me looking at switches. Testing direct traffic to that Mac exposed the interface problem.

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