Establish TCP Transport – you need to have IP reachibility between BGP peers. You need to think about BGP more as an application. It uses TCP port 179, unlike OSPF or EIGRP you dont enable the BGP on link level, you define the static peers that establish the TCP transport under the BGP process configuration.  Full Article…

It is important to note that when you are configuring in the SP the OSPF between the P routers they must have IPv4 reachibility between its loopback interfaces as these loopback interfaces will be used in MPLS tunnels as source IPs. Then the INE was quite shity regarding OSPF