Bridge & it's MAC address question

Bridge & it's MAC address question

[Dmitry Mishin]

Hi,

Could somebody explain, why bridge uses minimal MAC of the attached devices?
It makes this address instable, variable during bridge life-cycle, which is 
not good for DHCP. For example, I want to attach multiple virtual devices to 
one physical. Then, I need to make sure that after each virtual device 
addition, bridge addr is not changed and still addr of the physical device.  
Why not to use MAC of the first attached device?

-- 
Thanks,
Dmitry.

Re: Bridge & it's MAC address question

[Stephen Hemminger]

On Mon, 30 Oct 2006 15:27:14 +0300
Dmitry Mishin <dim@openvz.org> wrote:

> Hi,
> 
> Could somebody explain, why bridge uses minimal MAC of the attached devices?
> It makes this address instable, variable during bridge life-cycle, which is 
> not good for DHCP. For example, I want to attach multiple virtual devices to 
> one physical. Then, I need to make sure that after each virtual device 
> addition, bridge addr is not changed and still addr of the physical device.  
> Why not to use MAC of the first attached device?
> 

The bridge physical address is the minimum of all the attached devices.
This is done because the STP standard requires it.  You can reset it
to be the same as any of the attached devices. This will not cause a problem
unless using STP.

Re: Bridge & it's MAC address question

[Lennert Buytenhek]

On Mon, Oct 30, 2006 at 07:28:37AM -0800, Stephen Hemminger wrote:

> > Could somebody explain, why bridge uses minimal MAC of the attached devices?
> > It makes this address instable, variable during bridge life-cycle, which is 
> > not good for DHCP. For example, I want to attach multiple virtual devices to 
> > one physical. Then, I need to make sure that after each virtual device 
> > addition, bridge addr is not changed and still addr of the physical device.  
> > Why not to use MAC of the first attached device?
> 
> The bridge physical address is the minimum of all the attached devices.
> This is done because the STP standard requires it.  You can reset it
> to be the same as any of the attached devices. This will not cause a
> problem unless using STP.

You can in fact use any MAC address.  The STP standard recommends using
the minimum address, as that is deterministic, and so it doesn't depend
on the order in which you enslave subdevices.

Re: [Bridge] Bridge & it's MAC address question

[Stephen Hemminger]

On Fri, 15 Dec 2006 14:19:08 +0100
Lennert Buytenhek <buytenh@wantstofly.org> wrote:

> On Mon, Oct 30, 2006 at 07:28:37AM -0800, Stephen Hemminger wrote:
> 
> > > Could somebody explain, why bridge uses minimal MAC of the attached devices?
> > > It makes this address instable, variable during bridge life-cycle, which is 
> > > not good for DHCP. For example, I want to attach multiple virtual devices to 
> > > one physical. Then, I need to make sure that after each virtual device 
> > > addition, bridge addr is not changed and still addr of the physical device.  
> > > Why not to use MAC of the first attached device?
> > 
> > The bridge physical address is the minimum of all the attached devices.
> > This is done because the STP standard requires it.  You can reset it
> > to be the same as any of the attached devices. This will not cause a
> > problem unless using STP.
> 
> You can in fact use any MAC address.  The STP standard recommends using
> the minimum address, as that is deterministic, and so it doesn't depend
> on the order in which you enslave subdevices.

So should restriction be lifted?
Please update wiki page FAQ, or I'll do it

Re: [Bridge] Bridge & it's MAC address question

[Lennert Buytenhek]

[ dropped subscriber-only openvz.org list ]

On Fri, Dec 15, 2006 at 07:52:36AM -0800, Stephen Hemminger wrote:

> > > The bridge physical address is the minimum of all the attached devices.
> > > This is done because the STP standard requires it.  You can reset it
> > > to be the same as any of the attached devices. This will not cause a
> > > problem unless using STP.
> > 
> > You can in fact use any MAC address.  The STP standard recommends using
> > the minimum address, as that is deterministic, and so it doesn't depend
> > on the order in which you enslave subdevices.
> 
> So should restriction be lifted?

We should definitely allow users to override the MAC address of a
bridge interface.


> Please update wiki page FAQ, or I'll do it

Please do.

Re: Bridge & it's MAC address question

[Sven Köhler]

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>>>> The bridge physical address is the minimum of all the attached devices.
>>>> This is done because the STP standard requires it.  You can reset it
>>>> to be the same as any of the attached devices. This will not cause a
>>>> problem unless using STP.
>>> You can in fact use any MAC address.  The STP standard recommends using
>>> the minimum address, as that is deterministic, and so it doesn't depend
>>> on the order in which you enslave subdevices.
>> So should restriction be lifted?
> 
> We should definitely allow users to override the MAC address of a
> bridge interface.

Yes, i would love to have that feature.

I'm using a bridge for a Xen host machine. Interfaces of the Xen guests
are dynamically added to and removed from the bridge. At the moment, i
create and add a tun interface to the bridge, so the bridge has a valid
mac address at all.



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