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?
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.