I think I have figured out how to recreate this. This is happening on system with 2 different types of card. I think this really needs to be critical. I have a system which does not even have a eth2/eth3 interface. Yet, it keeps picking eth2 for juju-br0. 


$ sudo ifconfig
eth0      Link encap:Ethernet  HWaddr d4:ae:52:cb:c0:fa  
          inet addr:10.245.0.242  Bcast:10.245.63.255  Mask:255.255.192.0
          inet6 addr: fe80::d6ae:52ff:fecb:c0fa/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:162462 errors:0 dropped:0 overruns:0 frame:0
          TX packets:34639 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:233671235 (233.6 MB)  TX bytes:2994444 (2.9 MB)

juju-br0  Link encap:Ethernet  HWaddr fe:8b:6f:87:1e:61  
          inet6 addr: fe80::786a:19ff:fef7:9a/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:274 errors:0 dropped:0 overruns:0 frame:0
          TX packets:134 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:77656 (77.6 KB)  TX bytes:43740 (43.7 KB)

lo        Link encap:Local Loopback  
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:198 errors:0 dropped:0 overruns:0 frame:0
          TX packets:198 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:9900 (9.9 KB)  TX bytes:9900 (9.9 KB)

lxcbr0    Link encap:Ethernet  HWaddr 06:6b:1b:b9:20:f6  
          inet addr:10.0.3.1  Bcast:10.0.3.255  Mask:255.255.255.0
          inet6 addr: fe80::46b:1bff:feb9:20f6/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:8 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:648 (648.0 B)

veth2L1AU3 Link encap:Ethernet  HWaddr fe:8c:29:79:be:03  
          inet6 addr: fe80::fc8c:29ff:fe79:be03/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:110 errors:0 dropped:0 overruns:0 frame:0
          TX packets:213 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:32676 (32.6 KB)  TX bytes:68126 (68.1 KB)

veth7RKGAH Link encap:Ethernet  HWaddr fe:8b:6f:87:1e:61  
          inet6 addr: fe80::fc8b:6fff:fe87:1e61/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:131 errors:0 dropped:0 overruns:0 frame:0
          TX packets:202 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:40122 (40.1 KB)  TX bytes:62052 (62.0 KB)

virbr0    Link encap:Ethernet  HWaddr 9e:c7:cb:65:79:17  
          inet addr:192.168.122.1  Bcast:192.168.122.255  Mask:255.255.255.0
          UP BROADCAST MULTICAST  MTU:1500  Metric:1
          RX packets:0 errors:0 dropped:0 overruns:0 frame:0
          TX packets:0 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:0 
          RX bytes:0 (0.0 B)  TX bytes:0 (0.0 B)

       
$ cat /etc/network/interfaces
auto lo



iface eth3 inet dhcp

iface eth4 inet dhcp

iface eth5 inet dhcp

auto eth0

iface eth0 inet dhcp

iface eth1 inet dhcp

iface eth2 inet manual

auto juju-br0
iface juju-br0 inet dhcp
    bridge_ports eth2
$ sudo ifconfig eth0
eth0      Link encap:Ethernet  HWaddr d4:ae:52:cb:c0:fa  
          inet addr:10.245.0.242  Bcast:10.245.63.255  Mask:255.255.192.0
          inet6 addr: fe80::d6ae:52ff:fecb:c0fa/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:163321 errors:0 dropped:0 overruns:0 frame:0
          TX packets:34774 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000 
          RX bytes:233741731 (233.7 MB)  TX bytes:3012549 (3.0 MB)

$ sudo ifconfig eth2
eth2: error fetching interface information: Device not found