Sorry for messing up some of the VLAN unit allocation logic. I have one suggestion though:

Ah, I should have anticipated that. It's done, thank you.

This revision implements the test plan above as a proper set of ATF(7)/KYUA(1) test cases.
It also addresses the small nit in if_vlan.c (lowercase "802.1q" for consistency).

That's great news, thanks ! I'm currently catching up on Kyua & the ATF; I'll push some tests today.

I'll just slip in a small cosmetic update while I still can (MT_LAMBDA becomes MT_FILTER).

This updated revision implements the alternative solution discussed earlier. I've updated the test plan to account for the new parameter-based syntax, so I will not dwell on that here, but in a nutshell:

ifconfig svlan5 create vlandev em0 vlan 5 up
ifconfig vlan42 create vlandev svlan5 vlan 42 inet 10.5.42.1/24

becomes

ifconfig vlan5 create vlandev em0 vlan 5 vlanproto 802.1ad up
ifconfig vlan42 create vlandev vlan5 vlan 42 inet 10.5.42.1/24

Dot notation is supported, as long as everyone agrees that any interface with a dot in its name is a VLAN.

This is mostly a cosmetic update. It fixes the small issues (typos, etc...) raised in the previous revision. The idea is to have a clean base to compare the (upcoming) alternative implementation to.

Re "vlan"/"svlan" naming - I don't have extremely strong opinion on this, but it would be nice if we could still consider using the same name for both. "Svlan" looks a bit like low-level implementation detail somehow leaked to the upper layers.

For example, it can be implemented in the following fashion:
ifconfig vlan5 create vlan 5 vlandev em0 vlanproto 802.1ad <-- svlan
ifconfig vlan5.25 create vlan 5 vlandev vlan5 <-- 8021.q on top of 802.1ad
ifconfig em0.5 create vlanproto 802.1ad <-- svlan

Do you think it would be more confusing for users?

This updated revision improves support for dot notation.

Hmm.. parsing VLAN interface names is currently left to the kernel. It sounds like something ifconfig should handle instead, especially when non-trivial dot notation is involved (ie. ifconfig em0.x.y.z).
I mean, look at vlan_clone_create(), the first half of it is just about normalizing user input.

@rscheff I completely agree with your points. I probably should have given this more thought to begin with.
I will amend the patch accordingly later today.

Hi, thanks for considering this patch.

My mistake for not linting the man pages before submitting this revision.
Thanks for the heads-up !

• Fix broken ifconfig.8 syntax.
• Add svlan.4 and if_svlan.4 as symlinks to vlan.4.
• Slight update to the revision title.

There should be eventually. However, unless the general consensus is that
this should be an absolute requirement, it is not on the top of my priority
list at this time (there are other things I'd like to go through first,
such as proper support for 802.1ad hardware tagging).