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

Stephen Hemminger shemminger at linux-foundation.org
Thu Jun 30 09:22:58 PDT 2011

On Thu, 30 Jun 2011 15:05:36 +0200
Arnaud Fontaine <arnau at 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

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

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