VLAN problem on some em(4) cards

[Christoph Moench-Tegeder]

Hi,
while replacing a switch I ran into a problem with using 802.1q VLANs
(vlan(4)) on a em(4) card - that is, I could not get the card to work
with VLANs.
Details:
Software is FreeBSD 10.3-RELEASE i386 with a custom kernel (but has
em(4) and vlan(4) compiled in (not as a module)).
Hardware is a Soekris net6501-70, the interface in question is (pciconf)
em2 at pci0:10:0:0:        class=0x020000 card=0x00008086 chip=0x10d38086 rev=0x00
    vendor     = 'Intel Corporation'
    device     = '82574L Gigabit Network Connection'
    class      = network
    subclass   = ethernet

The setup I wanted to create was a "router on a stick", that is having
3 (for starters) tagged VLANs on that one link between router and
switch. I was not able to get any packet through to the switch,
while tcpdump shows the packets on the vlan interface (but not on
the underlying em interface - I understand that this could be the
result of hardware vlan processing). I could not ping any host beyond
the switch nor the switch's management interface itself, I did not
even receive ARP replies.
I tried both manual creation of the vlan devices and the rc.conf/
netif mechanism - with the very same results ("doesn't work").
Experimenting with the vlanhw* flags on the em interface did not
result in any change.

To make sure it's not the switch (or my own mistake), I took another
host with an em interface - this time, it was a
em0 at pci0:0:25:0:        class=0x020000 card=0x21ce17aa chip=0x15028086 rev=0x04
    vendor     = 'Intel Corporation'
    device     = '82579LM Gigabit Network Connection'
    class      = network
    subclass   = ethernet

(again 10.3-RELEASE, but amd64).
With this interface (connected to the very same switch port) I
had instantaneous success, so I guess the switch is ok and I'm
not totally mistaken in my doings.

Could anybody give a hint on how to further debug this or a pointer
to further reading?

Regards,
Christoph

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