Test on interfaces that do not support hardware VLAN insertion.
em0 <---> igc0
```
# ifconfig igc0 up
# tcpdump -nevi igc0
```
```
# ifconfig em0 -vlanhwtag -pcp
# ifconfig em0 inet 192.0.2.1/24
# arp -s 192.0.2.2 0:1:2:3:4:5
# ping -c1 -C3 192.0.2.2
```