Network Link down until switch on other side of network restarted
Networks are silly.
Problem: Link between two switches not working
Solution: Restart a switch unconnected to either of the problematic switches
The problem was apparently a one off, but I'd be interested if anyone has any idea what was happening. We have switches A to E connected like this by Gigabit ethernet (as well as some other switches branching off):
Links are copper, except for a fibre link from C to D. With everything plugged in, no device that was connected to B, C, D or E could talk to any device connected to A, or vice versa. However if I unplugged cable B-C or cable C-D, then suddenly the link A-B works again. Unplugging D-E, did not let A-B work, so the problem was narrowed down to switch D. After I restarted switch D, the link A-B worked fine again.
The switches were probably in a weird state from some network trouble earlier in the day: a computer had been continually spamming DHCP requests which caused the whole network to grind to a halt. However that had been stopped hours before - the A-B link problem appeared after the end of the school day when network traffic should have been reaching a minimum.
Has anyone seen anything like this before? Does anyone have any idea how it could happen? My best guess is something to do with Spanning Tree Protocol (which we have switched on, though I do not think there is any need for it to be on).