[Bridge] Bridge interface always down since Linux >= 2.6.39

[Bridge] Bridge interface always down since Linux >= 2.6.39

[Arnaud Fontaine]

Hello,

After upgrading my  kernel to 2.6.39 (I also  tested with 3.0.0-rc4), my
bridge interface  with only TAP interfaces  is always down  (the flag is
NO_CARRIER and  operational state is  DOWN) despite it was  working well
with  previous  versions of  the  kernel  (the  state was  LOWER_UP  and
operational state  was UNKNOWN). Before, running  the following commands
was enough to activate the bridge interface:

  1. brctl addbr br0
  2. ip link set br0 up

Only by attaching a real network interface (such as eth0) to the bridge,
it finally works well  but I would like to be able  to not only use real
network interfaces.

Any idea? Many thanks in advance.

Regards,
-- 
Arnaud Fontaine

Re: [Bridge] Bridge interface always down since Linux >= 2.6.39

[Stephen Hemminger]

On Thu, 30 Jun 2011 15:05:36 +0200
Arnaud Fontaine <arnau@milkypond.org> wrote:

> Hello,
> 
> After upgrading my  kernel to 2.6.39 (I also  tested with 3.0.0-rc4), my
> bridge interface  with only TAP interfaces  is always down  (the flag is
> NO_CARRIER and  operational state is  DOWN) despite it was  working well
> with  previous  versions of  the  kernel  (the  state was  LOWER_UP  and
> operational state  was UNKNOWN). Before, running  the following commands
> was enough to activate the bridge interface:
> 
>   1. brctl addbr br0
>   2. ip link set br0 up
> 
> Only by attaching a real network interface (such as eth0) to the bridge,
> it finally works well  but I would like to be able  to not only use real
> network interfaces.
> 
> Any idea? Many thanks in advance.
> 
> Regards,

There are two flags, IFF_UP - admin up/down and IFF_RUNNING carrier up/down
The IFF_UP is controlled manually, and IFF_RUNNING is what you are talking
about.

In older kernel versions, the bridge device always asserted carrier even
if all ports did not have carrier. Now it reflects the state of the underlying
device. In your case the problem is that the tap interfaces aren't reporting
carrier.

Re: [Bridge] Bridge interface always down since Linux >= 2.6.39

[Arnaud Fontaine]

Hi,

> There are  two flags, IFF_UP  - admin up/down and  IFF_RUNNING carrier
> up/down The IFF_UP is controlled manually, and IFF_RUNNING is what you
> are talking about.

Interesting, thank you very much for the explanation.

> In older  kernel versions, the  bridge device always  asserted carrier
> even if all  ports did not have carrier. Now it  reflects the state of
> the  underlying device.  In  your case  the  problem is  that the  tap
> interfaces aren't reporting carrier.

This  is what  I understood  by testing  with a  real  network interface
attached to the  bridge, however I was not sure  whether it was intended
or a regression.  In this  particular case, a tap interface is reporting
carrier when  an userspace program is  attached to it AFAIK,  so I guess
there  is  nothing else  I  can do  besides  of  attaching an  userspace
program, right?

Thanks for the quick response.

Cheers,
-- 
Arnaud Fontaine