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

Stephen Hemminger shemminger at linux-foundation.org
Tue Oct 12 09:44:08 PDT 2010

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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