Why the state of
lo interface is reported as
cat /sys/class/net/lo/operstate even though
ifconfig says it's running?
lo interface does not support
operstate (as some other network driver).
The value unknown is documentated as:
IF_OPER_UNKNOWN (0): Interface is in unknown state, neither driver nor userspace has set operational state. Interface must be considered for user data as setting operational state has not been implemented in every driver.
See here for a kernel patch that fixes this.