What does the LOWER_UP state mean?

[SkyHiRider]

When you use the ip link show command and a device has LOWER_UP state, what does that exactly mean. I understand the states UP and DOWN, which mean the device is online or offline. There's also the state UNKNOWN, which I am not sure what means either. The command can't get the info needed from the kernel (as in an error occured in the middle between the kernel and the command query or the kernel does not allow this information to be sent) or the kernel itself has some kind of bug or error that is effecting how the interface is working or is not working as intended or not at all? Cheers in advance.

[HROAdmin26]

Google: ip link lower_up

Attatched is a patch for the LOWER_UP flag in 'ip link/addr' output.
Useful to check if network cable is inserted or not.

[SkyHiRider]

Thanks for googling it up for me, somehow I didn't find it. So basically if the interface has the lower up flag, it means the network cable is plugged in right? What about the unknown state, is the device behaving oddly/ not guaranteed to work properly or is there a driver problem that prevents the kernel from getting all the necessary data about the device? I seem to have a device that is in the Unknown state but has both the Up and Lower_UP flags, how exactly does that effect the device?