I've been using a telco (CenturyLInk, midwest USA) modem setup in transparent bridging to my RB3011 for dsl service. Yesterday my service was upgraded to fiber, and the new modem (router?) provided refuses to recognize my credentials when placed in transparent bridging. After spending several hours with tech support their recommendation was to attach the routerboard directly to the ont using my PPPoE credentials. The problem is that all traffic through the ont must be tagged as VLAN201. I've read several threads on the forums, and several other tutorials about setting up tagging and can't understand enough to relate to my current use. What I want is eth1 connected to the ont, and all the other ports available as part of a 192.168.1.x network with the routerboard as 192.168.1.1. Any help would be appreciated, I can use the cli if necessary but would prefer Winbox.

First stay on MTU 1480 to be safe and then 1484. I have the new RB750Gr3 and default on all ports there was a vlan 1. I had to set them all to ignore or something else that it would not be used. Than I added a vlan, in your case 20, in interfaces and TAB vlan.

First, remove any IP addresses or DHCP-clients from ether1.
Then add a vlan interface to ether1: Now add or move your PPPoE-Client to this vlan: Adjust your firewall rules in Filter and NAT which reference ether1 to pppoe-out1.
Done.
-Chris