Is it possible to set up the switching chip in a routerboard RB750 (or GS) to do dot1q tunneling ? Dot1q tunneling means blindly prepending a chosen vlan tag into any ingress packet and striping it from egress ones thus tunneling the original vlan tags that came from the client side of the tunnel.
The switch rules allow to add new tags only if there is none present and bridging allows this only by explicitly naming all tunneled vlans as qinq, none of which can provide the transparency of simple dot1q tunnel.
And unless I am misunderstanding I don’t think it will work with the 750
Vlan-header option (configured in /interface ethernet switch port) sets the VLAN tag mode on egress port. This option works only with Atheros 8316 switch chip
Atheros8316 is present on RB493G(ether1+ether6-ether9, ether2-ether5), RB1200(ether1-ether5), RB450G(all ports with ether1 optional[more]), RB435G(all ports with ether1 optional[more]), RB750G and RB1100(ether1-ether5, ether6-ether10).
I think that even if i could use the vlan-header option then it probably would not be sufficient. Leave-as-is is for standard trunk-to-trunk ports. Always strip is for access ports and add if missing is for trunk ports with management. I do not know of a way in which these could be combined in order to tunnel dot1q packets.
What you need to do is basically prepend a vlan vlan to any ingress packet on a port regardless of weather it already has a tag or not (resulting in a packet with two tags if it already had one). You also need to strip the outermost vlan tag on egress regardless of weather it is the last one or there are more left.