[openstack-dev] How does instance's tap device mac address generate?

Neil Jerram Neil.Jerram at metaswitch.com
Thu Jun 11 10:59:14 UTC 2015

On 11/06/15 10:47, changzhi wrote:
> Hi, all.
> I create a vm and it's neutron port's mac address is
> "fa:16:3e:3f:02:ff". I see "fa:16:3e:3f:02:ff" inside vm when I run
> "ifconfig eth0". Why does vm's tap device's mac address is
> "fe:16:3e:3f:02:ff"? Why different between neutron port's mac address
> and tap device's mac address? Does libvirt create instance tap device
> and its mac address is generated randomly?
> Thx
> zhi

There are two MAC addresses, one at each end of the link between the 
host and the VM.

  Host              |           +-------------------+
                    |           |             VM    |
     tap12345-CD ------------------- eth0           |
  fa:16:3e:56:71:42 |           | fa:16:3e:3f:02:ff |
                    |           |                   |
                    |           +-------------------+

I believe that both of these addresses are randomly generated, although 
I'm not sure exactly which components do that.

Does that help at all?


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