Re: [Bridge] Why should static MAC address match one of the port MAC addresses - bug?

[Rolf Fokkens <rolf.fokkens@wanadoo.nl>]

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Hi All,

Despite of what Stephen explains, 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). No arp response or anything else.

So, Michiel's claim seems to be true: the bridge mac address should 
apparently be one of the bridge interfaces. But I don't see the logic, 
this seems like a bug to me.

kernel is Fedora kernel-2.6.35.9-64.fc14.x86_64.

Cheers,

Rolf
>
> ------------------------------------------------------------------------
>
>     * /To/: Michel Lammertink <mlammertink@xxxxxxxxx
>       <mailto:mlammertink@DOMAIN.HIDDEN>>
>     * /Subject/: Re: Why should static MAC address match one of the
>       port MAC addresses.
>     * /From/: Stephen Hemminger <shemminger@xxxxxxxxxxxxxxxxxxxx
>       <mailto:shemminger@DOMAIN.HIDDEN>>
>     * /Date/: Tue, 12 Oct 2010 09:44:08 -0700
>     * /Cc/: bridge@xxxxxxxxxxxxxxxxxxxxxxxxxx
>       <mailto:bridge@DOMAIN.HIDDEN>
>     * /In-reply-to/: <4CB46549.2070901@xxxxxxxxx
>       <mailto:4CB46549.2070901@DOMAIN.HIDDEN>>
>     * /Organization/: Linux Foundation
>     * /References/: <4CB46549.2070901@xxxxxxxxx
>       <mailto:4CB46549.2070901@DOMAIN.HIDDEN>>
>
> ------------------------------------------------------------------------
> On Tue, 12 Oct 2010 15:40:25 +0200 Michel Lammertink<mlammertink@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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