FortiSwitch disconnected but operational
There have been a few similar posts, but none with the same constellation (OS level and topology):
We have a single FortiGate 100D running FortiOS 5.6.3 managing a stack of two FortiSwitch 124E with S124EN-v3.6.3-build4269.
FortiGate FortiLink LAG using Ports 12 and 13 connecting to Ports 23 and 24 of switch #1 (copper, no split-interface).
ISL (fiber optic) between Switch #1 and Switch #2 on ports 25 and 26 (25 on 25 and 26 on 26). (no direct link between Switch #2 and the FortiGate possible).
Switch #1 shows as "disconnected" and cannot be configured anymore. Switch #2 is functioning fine.
We are going to schedule a maintenance window and reboot the switches, but i would like to know how this could have happened and how to prevent it in the future.
Diagnostic: I can ping and telnet to both switches using the automatic FortiLink addresses (169.254.1.x), the disconnect happened more than 7 days ago (it's a new environment, no monitoring yet) so the event logs doesnt show anything anymore.
Diag status on switch #1 just confirms that there is no connection. Configuration is otherwise ok.
Since the Switch #2 is working correctly even though it is connected through Switch #1, i do not want to do any disruptive debugging on the "disconnected" switch. Is there any way to re-start the connection that does not disrupt connectivity for the connected devices?