I first setup Omina behind the ISP router and it was working.
Then plugged the cable directly in Omnia and fixed the VLAN (with help). eth1.101 got the IP number via DHCP and the route was setup but no traffic got through - not even ping 8.8.8.8 .
As the IP was set there must have been some traffic and I looked at with
tcpdump -i eth1
and while using tcpdump the traffic suddenly got through.
root@turris:~# grep promisc /var/log/messages
2018-10-27 08:18:36 info kernel[]: [ 22.604422] device eth0 entered promiscuous mode
2018-10-27 08:18:36 info kernel[]: [ 22.624336] device eth2 entered promiscuous mode
2018-10-27 08:18:36 info kernel[]: [ 23.115224] device wlan1 entered promiscuous mode
2018-10-27 08:18:36 info kernel[]: [ 25.445318] device wlan0 entered promiscuous mode
2018-10-27 08:19:03 info kernel[]: [ 60.241006] device eth1 entered promiscuous mode
The last line was when I started tcpdump.
The work around was to add a fix to /etc/rc.local:
ifconfig eth1 promisc
to put it into promiscous mode during boot.
(Bad hack: rc.local is somehow deprecated)
The question is: What went wrong? What could caused eth1 not to enter promiscous mode which I expect it has to?