Re: Forced lladdr change with bridge - or not?

[bdschuym@telenet.be]

You're right. The Linux bridge does just what the 802.1D standard recommends (http://standards.ieee.org/getieee802/download/802.1D-2004.pdf):
"7.12.5 Unique identification of a bridge
A unique 48-bit Universally Administered MAC Address, termed the Bridge Address, shall be assigned to
each Bridge. 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."

Apparently you can get around this problem, see (I didn't verify):
http://backreference.org/2010/07/28/linux-bridge-mac-addresses-and-dynamic-ports/

cheers,
Bart

----- Originele e-mail  -----
Van: "Pascal Hambourg" <pascal.mail@plouf.fr.eu.org>
Aan: "Jan Engelhardt" <jengelh@medozas.de>
Cc: "Bart De Schuymer" <bdschuym@pandora.be>, netfilter@vger.kernel.org
Verzonden: Zondag 22 augustus 2010 22:53:04 GMT +01:00 Amsterdam / Berlijn / Bern / Rome / Stockholm / Wenen
Onderwerp: Re: Forced lladdr change with bridge - or not?

Hello,

Jan Engelhardt a écrit :
> On Sunday 2010-08-22 18:55, Bart De Schuymer wrote:
>>> # brctl addbr br0
>>> # brctl addif br0 tap1
>>> # brctl addif br0 tap2
>>> # ip a
>>> 196: tap1: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 500
>>>    link/ether 9a:17:c4:65:e9:76 brd ff:ff:ff:ff:ff:ff
>>> 197: tap2: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN qlen 500
>>>    link/ether ce:61:28:5a:b7:93 brd ff:ff:ff:ff:ff:ff
>>> 198: br0: <BROADCAST,MULTICAST> mtu 1500 qdisc noop state DOWN     link/ether
>>> 9a:17:c4:65:e9:76 brd ff:ff:ff:ff:ff:ff
>>>
>>> Is this behavior normal that the lladdrs of all but the first brport
>>> remain unchanged? If so, what is the purpose of changing the lladdr on the
>>> first brport?
>>>  
>> I don't understand your question,

Me neither : according to your output, the MAC address of neither port
has changed.

> but the Linux bridge code assigns the MAC
>> address of its first-added port to the virtual bridge device.
> 
> 1. Why does it do that,

The bridge interface must have a MAC address, so why not pick up one in
thoses of its ports ?

> 2. Why only the first port?

IME, the bridge picks up the lowest MAC address of its ports.
This has an advantage : the bridge MAC address ends up the same for a
same set of ports regardless of the order in which the ports are added.
But this also has a big disadvantage : the bridge MAC address can change
whenever you add or remove a port, breaking ARP for a while, changing
link-local and stateless autoconfigured IPv6 addresses...