netdev - Bridge stays down until a port is added

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Message-ID: <20110811070659.GA21307@torres.zugschlus.de>
Date:	Thu, 11 Aug 2011 09:06:59 +0200
From:	Marc Haber <mh+netdev@...schlus.de>
To:	netdev@...r.kernel.org
Subject: Bridge stays down until a port is added

Hi,

since a few kernel versions, I think since 2.6.39, I have noticed that
a bridge interface stays in NO CARRIER state unless the first port is
added:

$ sudo brctl show
bridge name     bridge id               STP enabled     interfaces
br0             8000.000000000000       no
$ ip a show dev br0
9: br0: <NO-CARRIER,BROADCAST,MULTICAST,UP> mtu 1500 qdisc noqueue state DOWN
    link/ether 00:00:00:00:00:00 brd ff:ff:ff:ff:ff:ff
    inet6 2001:db8::1/64 scope global tentative
       valid_lft forever preferred_lft forever
$ ping6 2001:db8::1
PING 2001:db8::1(2001:db8::1) 56 data bytes
^C
--- 2001:db8::1 ping statistics ---
3 packets transmitted, 0 received, 100% packet loss, time 2016ms

$ sudo modprobe dummy
$ sudo brctl addif br0 dummy0
$ sudo ip link set dev dummy0 up
$ ping6 2001:db8::1
PING 2001:db8::1(2001:db8::1) 56 data bytes
64 bytes from 2001:db8::1: icmp_seq=1 ttl=64 time=0.019 ms
64 bytes from 2001:db8::1: icmp_seq=2 ttl=64 time=0.023 ms
^C
--- 2001:db8::1 ping statistics ---
2 packets transmitted, 2 received, 0% packet loss, time 999ms
rtt min/avg/max/mdev = 0.019/0.021/0.023/0.002 ms
$ ip a show dev br0
9: br0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP
    link/ether da:1c:11:bc:3e:54 brd ff:ff:ff:ff:ff:ff
    inet6 2001:db8::1/64 scope global
       valid_lft forever preferred_lft forever
    inet6 fe80::d81c:11ff:febc:3e54/64 scope link
       valid_lft forever preferred_lft forever
$

Is that a feature? If so, why does the interface stay pingable after
removing the dummy0 interface from the bridge?

Can I somehow get the old behavior back, that the bridge is
immediately up, and addresses configured on the bridge interfaces are
immediately pingable?

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?

Greetings
Marc

-- 
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Marc Haber         | "I don't trust Computers. They | Mailadresse im Header
Mannheim, Germany  |  lose things."    Winona Ryder | Fon: *49 621 72739834
Nordisch by Nature |  How to make an American Quilt | Fax: *49 3221 2323190
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