Using ip, what does LOWER_UP mean?

Question

When using the terminal tool ip, there is a number of flags for every interface.

Example: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000

The meaning of BROADCAST, MULTICAST and UP is clear, but what does LOWER_UP mean? I tried to google for this, but I didn't find a clear answer. When another host is connected to this link, then state goes to UP, when it disconnects, state goes to DOWN and LOWER_UP disapears. Does this have something to do with the connection state?

EDIT:

I have found another interesting fact. When I set the connected interface to down (via ip link set eth0 down) on the other host (cable stays connected), the output changes to

1: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000

Then the UP flag is still present, but not the LOWER_UP. Shouldn't it be the opposite way?

Answer 1

LOWER_UP is a physical layer link flag (the layer below the network layer, where IP is generally located). LOWER_UP indicates that an Ethernet cable was plugged in and that the device is connected to the network.

LOWER_UP differs from UP, which additionally requires the network interface to be enabled.

Answer 2

It's described in man netdevice(7), but not much meaningful to me:

IFF_LOWER_UP      Driver signals L1 up (since Linux 2.6.17) 

These comments are from linux/if.h header file (in enum net_device_flags).