Comment 5 for bug 1029273

I don't know much about the working of vlans either. I simply use them :-)

Looking at the ifupdown mechanism it seems to work like this:

You just configure the vlan in /etc/network/interfaces like: iface eth0.1 inet dhcp

When you then enter 'ifup eth0.1', the system configures eth0 and then sets on top the virtual interface eth0.1 Afterwards it starts the dhclient on the virtual interface. So, the end configuration of an "ifup eth0.1" looks like this (sorry, from German system):

eth0 Link encap:Ethernet Hardware Adresse 00:1f:16:0d:7b:41
          inet6-Adresse: fe80::21f:16ff:fe0d:7b41/64 Gültigkeitsbereich:Verbindung
          UP BROADCAST RUNNING PROMISC MULTICAST MTU:1500 Metrik:1
          RX packets:1468333 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1453849 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:1000
          RX-Bytes:919300549 (919.3 MB) TX-Bytes:629072382 (629.0 MB)
          Interrupt:20 Speicher:f2700000-f2720000

eth0.1 Link encap:Ethernet Hardware Adresse 00:1f:16:0d:7b:41
          inet Adresse:172.17.2.103 Bcast:172.17.2.127 Maske:255.255.255.128
          inet6-Adresse: fe80::21f:16ff:fe0d:7b41/64 Gültigkeitsbereich:Verbindung
          UP BROADCAST RUNNING MULTICAST MTU:1500 Metrik:1
          RX packets:292825 errors:0 dropped:0 overruns:0 frame:0
          TX packets:190175 errors:0 dropped:0 overruns:0 carrier:0
          Kollisionen:0 Sendewarteschlangenlänge:0
          RX-Bytes:342938624 (342.9 MB) TX-Bytes:29941003 (29.9 MB)

So, the parent device eth0 is up but has no IP address. The virtual device eth0.1 has an IP address. Same applies to the routes:

Ziel Router Genmask Flags Metric Ref Use Iface
default pbx.home 0.0.0.0 UG 100 0 0 eth0.1
link-local * 255.255.0.0 U 1000 0 0 eth0.1

(not sure what the link-local is good for)

Network Manager on the other hand does not do anything with eth0.1. It just configures eth0 and start the dhclient on eth0, although it reads its configuration from the same /etc/network/interfaces file. So, either it doesn't know how to handle vlans at all or it does not read /etc/network/interfaces correctly. But 0.9.4 is said to have vlan support.

Would it be better to go upstream with this question? However, those guys there deal with a different configuration system from rhel. They might not understand the Ubuntu implementation...