Configure KNX interfaces as ROUTER means, that they all communicate over a multicast IP address (default 224.0.23.12) together. This means, every ROUTER sends and listens at this IP without a hardwired connection.
In opposite, TUNNEL connections are hardwired to one endpoint -> A KNX interfaces as TUNNEL has a hardwired connection to a specific other KNX interface and its specific IP.
So configure KNXD AND openHAB (and other Interfaces) as ROUTER and all are happy together over the multicast IP. 
PS: Think about, that every ROUTER should have a unique address! (in openHAB knx:busaddr=15.15.249)