Comment 1 for bug 1305271

Confirmed via testing on a seperate VM.

Looking at the source of ifupdown-0.7~beta2ubuntu11/ifupdown.nw the following change appears to be the cause:

ip link set dev %iface% [[mtu %mtu%]] [[address %hwaddress%]] up

...

Yes, package compiled without that line and interface now comes up. why...

OK, the "up" line above is run _before_ the up lines in /etc/network/interfaces, at which point the vlan doesn't exist.

Modifying the interfaces stanza as follows resolves the issue:

# VLAN 3
auto reporter1
iface reporter1 inet manual
        pre-up ip l s dev intpub up
        pre-up vconfig add intpub 3
        pre-up ip l s dev intpub.3 name reporter1
        up ip l s dev reporter1 up
        up ip a a xx.xx.xx.xx/29 dev reporter1
        up ip a a 2403:9800:xx:xx::1/64 dev reporter1
        down ip a del xx.xx.xx.xx/29 dev reporter1
        down ip l s dev reporter1 down
        down vconfig rem reporter1

So, not sure if this is a bug exactly, but the change made in the latest patch DOES break existing functionality while attempting to add new features, this is (in my opinion) unacceptable for a stable version package update.