What is the second "state" in "ip link show" output

Question

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN mode DEFAULT group default 
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00

The UP between the brackets means my link is operationnally up. But what does the state UNKNOWN mean ?

I went through this doc but could not find anything related to it.

Answer 1

Well, lo is a loopback device that doesn't have a hardware-relatable state, whereas eth0 does.

superuser@hermes:~$ ip link show up
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 16436 qdisc noqueue state UNKNOWN
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP qlen 1000
    link/ether 0a:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff

Answer 2

The first one means the interface is administratively/logically up. The second one means it is acutally ready to send or receive packets. You can confirm this by "Link detect" field in ethtool