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

2015-06-11 Thread changzhi 
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__
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Re: [openstack-dev] How does instance's tap device mac address generate?

2015-06-11 Thread Neil Jerram 

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?

Thanks,
Neil

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

2015-06-11 Thread Radek Smigielski 
 On Thursday, 11 June 2015, 12:00:46, Neil Jerram neil.jer...@metaswitch.com 
 wrote:
  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?
 
 Thanks,
 Neil



In neutron.conf you've got base_mac option and fa:16:3e is the default value.

# Base MAC address. The first 3 octets will remain unchanged. If the
# 4h octet is not 00, it will also be used. The others will be
# randomly generated.
# 3 octet
# base_mac = fa:16:3e:00:00:00
# 4 octet
# base_mac = fa:16:3e:4f:00:00





Cheers,
Radosław Śmigielski

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