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Date: Wed, 12 Mar 2008 08:14:45 -0700
From: Stephen Hemminger <shemminger@linux-foundation.org>
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Subject: Re: [Bridge] Question about using brctl and changing MAC addresses
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To: Andy Gospodarek <andy@greyhouse.net>
Cc: Philip Prindeville <philipp_subx@redfish-solutions.com>, bridge@linux-foundation.org

On Wed, 12 Mar 2008 10:46:48 -0400
"Andy Gospodarek" <andy@greyhouse.net> wrote:

> On Tue, Mar 11, 2008 at 11:29 PM, Philip Prindeville
> <philipp_subx@redfish-solutions.com> wrote:
> >
> > Stephen Hemminger wrote:
> >  > On Mon, 10 Mar 2008 11:05:53 -0700
> >  > Philip Prindeville <philipp_subx@redfish-solutions.com> wrote:
> >  >
> >  >
> >  >> I'm using Linux 2.6.20 (yes, I'll be upgrading to 2.6.24 as soon as it's
> >  >> stable).
> >  >>
> >  >> I have 4 VIA rhine ethernet controllers on my Soekris net5501, plus an
> >  >> ADSL PCI card (a Sangoma S-518) that runs in AAL5-SNAP mode, so it looks
> >  >> like an Ethernet controller.
> >  >>
> >  >> I'm trying to bridge eth0 and w1ad (the ADSL interface) into "br0".  And
> >  >> I'm trying to force the traffic on br0 to have the MAC address that my
> >  >> ISP insists I use (i.e. that of the crappy little Westell 6100 modem
> >  >> they sent me).
> >  >>
> >  >> The issue is the following.  If I do:
> >  >>
> >  >> ifconfig w1ad down hw ether x:x:x:x:x:x up
> >  >>
> >  >> followed by either:
> >  >>
> >  >> brctl addbr br0
> >  >> brctl addif br0 eth0
> >  >> brctl addif br0 w1ad
> >  >>
> >  >> or:
> >  >>
> >  >> brctl addbr br0
> >  >> brctl addif br0 w1ad
> >  >> brctl addif br0 eth0
> >  >>
> >  >> the bridge ends up taking the MAC address of eth0 either way, which I
> >  >> don't get (how does it decide which to use?).
> >  >>
> >  >> I can clone the same MAC address to both eth0 and w1ad, but I don't know
> >  >> if that would cause me any problems or not (I haven't read the STP spec
> >  >> in about 15 years).
> >  >>
> >  >>
> >  >
> >  > Having the same mac address on both just is going to be problematic (impossible
> >  > to do STP), so don't if you don't have to.
> >  > But you can set address of bridge and eth0 to any address you want. So set these
> >  > to the ISP assigned address.  You probably will have to do it after bridge
> >  > is created.
> >  >
> >  > brctl addbr br0
> >  > brctl addif br0 w1ad br0
> >  > ifconfig eth0 hw ether x:x:x:x:x:x up
> >  > ifconfig w1ad up
> >  > ifconfig br0 hw ether x:x:x:x:x:x up
> >  >
> >  > ifconfig br0 A.B.C.D netmask 255.255.255.0
> >  >
> >
> >  That seems to not work.  I get:
> >
> >  # ifconfig br0 hw ether "00:18:3A:55:15:35"
> >
> > ifconfig: SIOCSIFHWADDR: Cannot assign requested address
> >  #
> >
> >  Any ideas why?  I'm doing this before an IP address has even been
> >  assigned...

Assign the hardware address of the bridge was only added in later kernels.