While working on T319184 myself and @aborrero hit an issue which prevented networking from properly starting on WMCS hosts (after moving them to a single NIC handoff.)

The problem was caused because in this case the physical interface is not a member of any bridge, but vlan sub-interfaces of it are. The bug is that a script from bridge-utils gets run when the child sub-interface is processed, disabling IPv6 on the parent:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=989162#35

In our case this stopped the primary interface from having any IPs set, as the commands to add the v6 token and address on it failed, causing the whole ifup script to crash.

Arturo has patched this via puppet to overcome the issue for affected WMCS hosts:

https://gerrit.wikimedia.org/r/c/operations/puppet/+/837078

I don't believe we currently have any other hosts with this exact combination of device types, so we should not encounter it elsewhere for now.

@aborrero is in discussion with the maintainers of the bridge-utils package in Debian to upstream these changes to it. If that doesn't happen we could alternatively package our own version with the fix. Or indeed this might just be another reason to look beyond ifupdown to systemd-networkd or alternate network init framework.

Creating this task for visibility and to track the issue.