Hello wise crowd!, I seek your help. I setup my mikrotik with VLAN 77 on it's eth1 as you can see on diagrams B and C and a WiMAX cell with VLAN 77 on its ethernet port (tagged only). If I check User Service Tag on my mikrotik VLAN (dawing B) the communication is lost, if I uncheck the User Service Tag (drawing C) pings start to work again. What could be the problem, what exactly does the User Service Tag do? is it Q in Q?

Yes, the user service tag is q-in-q.

Yes, the user service tag is q-in-q.

Hello fewi, thanks again. How do I tag a vlan in mikrotik?

Connect the ethernet interface of a RouterOS device to another device that does dot1q. Create a VLAN interface with the ethernet port as the parent, and apply the VLAN id you want (like creating a dot1 sub-interface on a Cisco router). Traffic from the VLAN network will go out the ethernet interface with the VLAN id tagged.

Connect the ethernet interface of a RouterOS device to another device that does dot1q. Create a VLAN interface with the ethernet port as the parent, and apply the VLAN id you want (like creating a dot1 sub-interface on a Cisco router). Traffic from the VLAN network will go out the ethernet interface with the VLAN id tagged.

Thank you fewi, let me be sure of what I will do: I will create a VLAN, then associate it with a physical interface. That interface will be connected to a tagged port (wimax cell or switch) and since the switch/wimax cell port is tagged all my traffic entering and exiting the router will be tagged right?

Do not use "User Service Tag".
That's like VLAN inside VLAN.