When using the terminal tool <code>ip</code>, there is a number of flags for every interface. Example: <code>eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc mq state UP group default qlen 1000 </code> The meaning of <code>BROADCAST</code>, <code>MULTICAST</code> and <code>UP</code> is clear, but what does <code>LOWER_UP</code> 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 <code>UP</code>, when it disconnects, state goes to <code>DOWN</code> and <code>LOWER_UP</code> 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 <code>ip link set eth0 down</code>) on the other host (cable stays connected), the output changes to <code>1: eth0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc mq state DOWN group default qlen 1000</code> Then the <code>UP</code> flag is still present, but not the <code>LOWER_UP</code>. Shouldn't it be the opposite way?

It's described in man <code>netdevice(7)</code>, but not much meaningful to me: <pre class="prettyprint"><code>IFF_LOWER_UP Driver signals L1 up (since Linux 2.6.17) </code></pre> These comments are from linux/if.h header file (in <code>enum net_device_flags</code>).

Using ip, what does LOWER

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?

What is QLEN in Linux?

qlen is the default transmit queue length of the device measured in packets. The interface flags are summarized in the angle brackets. UP -- the device is turned on. It is ready to accept packets for transmission and it may inject into the kernel packets received from other nodes on the network.

What is Valid_lft?

valid_lft indicates the lifetime of the IP address. preferred_lft indicates the lifetime of the IP address, which can be used as a source IP address (IPv6 only). Let's say you want to use an IP address for 10 seconds. ip addr add 192.168.4.244/23 dev eth0 valid_lft 10 preferred_lft 10. 192.168.

How do I enable a network port in Linux?

How to Enable a Network Interface. The “up” or “ifup” flag with interface name (eth0) activates a network interface if it is not inactive state and allowing to send and receive information. For example, “ifconfig eth0 up” or “ifup eth0” will activate the eth0 interface.

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.

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).

Using ip, what does LOWER_UP mean?

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