[Bridge] Why should static MAC address match one of the port MAC addresses.

[Rolf Fokkens]

Hi all,

I'm having the problem Michiel describes as well. I have a bridge br2 
containing 1 (but potentially more) tap interface for kvm. Windows 7 VM 
doesn't like the bridge mac address to change, so I try to make it fixed.

ifconfig br2 hw ether 02:00:00:00:01:02

Interface br2 becomes instantly unreachable from the VM (via the tap 
interface).

So, Michiel's claim still seems to be true: the bridge mac address 
should apparently be one of the bridge interfaces.

I don't see the logic, this seems like a bug to me.

Cheers,

Rolf

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    * /To/: Michel Lammertink <mlammertink at xxxxxxxxx
      <mailto:mlammertink at DOMAIN.HIDDEN>>
    * /Subject/: Re: Why should static MAC address match one of the port
      MAC addresses.
    * /From/: Stephen Hemminger <shemminger at xxxxxxxxxxxxxxxxxxxx
      <mailto:shemminger at DOMAIN.HIDDEN>>
    * /Date/: Tue, 12 Oct 2010 09:44:08 -0700
    * /Cc/: bridge at xxxxxxxxxxxxxxxxxxxxxxxxxx <mailto:bridge at DOMAIN.HIDDEN>
    * /In-reply-to/: <4CB46549.2070901 at xxxxxxxxx
      <mailto:4CB46549.2070901 at DOMAIN.HIDDEN>>
    * /Organization/: Linux Foundation
    * /References/: <4CB46549.2070901 at xxxxxxxxx
      <mailto:4CB46549.2070901 at DOMAIN.HIDDEN>>

------------------------------------------------------------------------

On Tue, 12 Oct 2010 15:40:25 +0200 Michel Lammertink<mlammertink at xxxxxxxxx>  wrote:

>/  Hi all,/
>/  /
>/  I've read many messages on this mailing list with the question about/
>/  setting a static MAC address on the bridge interface, but two questions/
>/  remain:/
>/  /
>/   1. Why should the MAC address of the bridge match the address of one of/
>/  its interfaces? Setting the MAC address to a different value does not/
>/  generate an error, but the bridge is not working properly (Local data/
>/  does not arrive)./

No longer a requirement. It was just a good way to ensure a good
address that was unique, and not being used by other systems.

>/   2. Why does the bridge by default choose the "lowest" MAC address,/
>/  instead of the MAC address of the first port added, as is proposed by/
>/  the standard [1]?/

It was a convenience in original design to choose a consistent value.
Since ports can be added in any order, using the lowest seemed like
a good idea for the original author. This has been maintained for
backwards compatibility.

>/  [1] IEEE802.1D, Par. 7.12.5./
>/      Quote: "The Bridge Address may be the individual MAC Address of a/
>/  Bridge Port, in which case, use of the address of the lowest numbered/
>/  Bridge Port (Port 1) is recommended."/
>/  /
>/  Thanks in advance,/
>/  /
>/  Michel./

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