I have RB435G and RB751G-2HnD. What I am trying to do is to connect (bridge) these to routers together using EoIp tunnel. There was said in manual create EoIP tunnel and bridge it together on both ends. On RB435G I added ethernet interface (which is connected with wire to RB751) and added EoIP tunnel. I also created bridge on RB751 added ether4 interface and when I finally adding Eoip tunnel there is an error message:Couldn't add New Bridge Port -eoip-tunnel1 is already slave (6). What am I doing wrong or there is any limitations on RB751G-2HnD?
Idea of this setup is to connect two wireless access points together so users can move seamlessly between both. Maybe there is another way to solve this?

You should not add the interface used for the eoip tunnel into the bridge, but the remainder of the interfaces.
If interface ether4 is connected on both ends to interconnect the RouterBoards and to setup eoip interface, put all ports except ether4 into the bridge, together with the eoip interface

Sounds reasonable, so I just tried to join in bridge EoIp tunnel1 and wlan1 on RB751 and got same error when trying to add EoIp tunnel despite ether4 was left alone.

Please share the config of both your devices to give proper advice

Now the problem is solved. Previously I had been entered some manual route setting in IP/Routes affecting EoIP tunnel interface. I deleted this record and can join interfaces together now. Thanks Rudios.

Good to hear you got it working!