title: what role does network bridge `docker0` play in k8s with flannel
body: <blockquote>
    <p>k8s version: v1.10.4<br>
    flannel version: v0.10.0<br>
    docker version v1.12.6 </p>
  </blockquote>
  
  <p>when i use command <code>brctl show</code> on node,it shows as bellow:</p>
  
  <pre><code>[root@node03 tmp]# brctl show
  bridge name bridge id       STP enabled interfaces
  cni0        8000.0a580af40501   no      veth39711246
                                          veth591ea0bf
                                          veth5b889fed
                                          veth61dfc48a
                                          veth6ef58804
                                          veth75f5ef36
                                          vethc162dc8a
  docker0     8000.0242dfd605c0   no
  </code></pre>
  it shows that the vethXXX are binding on network bridge named cni0, but when i use command `ip addr`,it shows :
  
  <pre><code>[root@node03 tmp]# ip addr |grep veth
  6: veth61dfc48a@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  7: veth591ea0bf@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  9: veth6ef58804@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  46: vethc162dc8a@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  55: veth5b889fed@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  61: veth75f5ef36@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP 
  78: veth39711246@if3: &lt;BROADCAST,MULTICAST,UP,LOWER_UP&gt; mtu 1450 qdisc noqueue master cni0 state UP
  </code></pre>
  these veth are all binding on `if3` ,but `if3` is not cni0.it is `docker0`
  
  <pre><code>3: docker0: &lt;NO-CARRIER,BROADCAST,MULTICAST,UP&gt; mtu 1500 qdisc noqueue state DOWN 
  </code></pre>
  
  <p>it seems that network bridge <code>docker0</code> is useless, but <code>ip addr</code> shows that all veth device are binding on it . what role does network bridge <code>docker0</code> play in k8s with flannel? thanks</p>