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

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Message-ID: <20110820093059.75c37a1f@nehalam.ftrdhcpuser.net>
Date:	Sat, 20 Aug 2011 09:30:59 -0700
From:	Stephen Hemminger <shemminger@...tta.com>
To:	Marc Haber <mh+netdev@...schlus.de>
Cc:	netdev@...r.kernel.org, Sven-Haegar Koch <haegar@...net.de>
Subject: Re: Bridge stays down until a port is added

On Sat, 20 Aug 2011 11:47:12 +0200
Marc Haber <mh+netdev@...schlus.de> wrote:

> Hi,
> 
> I was a little bit confused. The problem is that with IPv6 an IP
> address configured on a bridge which is still in the NO-CARRIER state
> will never leave tentative state and will thus not get useable.
> 
> On Fri, Aug 12, 2011 at 02:22:27PM +0200, Sven-Haegar Koch wrote:
> > For me (using kernel 3.0.0) it seems to work as I expect it:
> > 
> > aurora:~# brctl addbr br0
> > aurora:~# ifconfig br0 192.168.254.1 netmask 255.255.255.0 up
> > aurora:~# ping 192.168.254.1
> > PING 192.168.254.1 (192.168.254.1) 56(84) bytes of data.
> > 64 bytes from 192.168.254.1: icmp_req=1 ttl=64 time=0.087 ms
> 
> Now try it with IPv6.
> 

The problem is that IPv6 Duplicate Address Detection needs to
work. This is not a simple problem.  If the bridge asserted
carrier with no ports then:

1. IPv6 address assigned and IPv6 decides it is okay.
2. Port added later
3. Another system has the same address.
*broke*

If you want to avoid DAD, then you can configure disable DAD
by setting /proc/sys/net/ipv6/conf/br0/accept_dad to 0
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