Linux Ethernet Bridging - Oct 2010 - [Bridge] Why should static MAC address match one of the port MAC addresses.

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).

 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]?

[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.

On Tue, 12 Oct 2010 15:40:25 +0200
Michel Lammertink <mlammertink at ti-wmc.nl> 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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> Bridge mailing list
> Bridge at lists.linux-foundation.org
> https://lists.linux-foundation.org/mailman/listinfo/bridge

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