netdev - Re: Bridge stays down until a port is added

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Message-ID: <20110917204257.GA2636@torres.zugschlus.de>
Date:	Sat, 17 Sep 2011 22:42:57 +0200
From:	Marc Haber <mh+netdev@...schlus.de>
To:	netdev@...r.kernel.org
Subject: Re: Bridge stays down until a port is added

On Thu, Aug 11, 2011 at 08:17:06AM -0700, Stephen Hemminger wrote:
> On Thu, 11 Aug 2011 09:06:59 +0200
> Marc Haber <mh+netdev@...schlus.de> wrote:
> > New new behavior is somewhat unhandy when one uses the bridge address
> > for services that the host offers, to save on IP addresses and
> > networks (for example, when one has only a single IP address and a
> > single additional network), since one has to take extra measures to
> > have the addresses on the bridge interface reachable.
> > 
> > Or am I doing things wrong?
> 
> The goal is to make the bridge behave the same as a vlan or
> a physical device.  Could you explain better what the application(s)
> would expect.

The situation of a bridge is quite different from a physical ethernet
interface. The physical interface is usually connected to a switch and
is thus immediately up, even if no systems other than the switch are
online.

In the case of a bridge on a virtualization host used to connect
virtual machines, the bridge only goes up after the first VM was
started.

In the IPv6 SLAAC case, the host tries start its radvd on the bridge
well before the first VM comes up, the radvd barfs since the interface
is not up, does not come up, and the VMs are without network. In this
case, it is needed to force the bridge into an UP state earlier so
that radvd can start.

If people are supposed to add a dummy interface to the bridge if they
want to use ipv6, this should be documented. And it's ugly.

Greetings
Marc

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