title: Using ip, what does LOWER_UP mean?
body: <p>When using the terminal tool <code>ip</code>, there is a number of flags for every interface.</p>
  
  <p>Example:
  <code>eth0: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1500 qdisc mq state UP group default qlen 1000
  </code></p>
  
  <p>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?</p>
  
  <p><strong>EDIT:</strong></p>
  
  <p>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</p>
  
  <p><code>1: eth0: &lt;NO-CARRIER,BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc mq state DOWN group default qlen 1000</code></p>
  
  <p>Then the <code>UP</code> flag is still present, but not the <code>LOWER_UP</code>. Shouldn't it be the opposite way?</p>