Unmanaged Hub Outage on Cisco Catalyst: Err-Disabled Ports, Loops, and Storm Control
A few users on one office floor reported that the network was down. Their PCs could not ping the default gateway. The switch port facing the unmanaged hub they were plugged into was in the err-disabled state. This post covers how the problem was isolated, what the switch can and cannot tell you about the cause, and which Catalyst features limit the damage when an unmanaged hub misbehaves. Quick Summary Observed: 3 to 5 users in one area lost network access. The gateway was unreachable from their PCs. The access switch port connected to their hub was err-disabled. Fix that worked: Unplug every device from the hub, keep only the uplink, clear the err-disabled state, then reconnect the devices one at a time and watch the port. Not recorded: The exact err-disable reason was not captured, and the switch software version was not noted. This post does not claim a root cause for that incident. Why it matters: A hub cannot be monitored or protected, so the switch port it hang...