Techniques for utilizing a Software-Defined-Networking (SDN) controller and/or a Data Center Network Manager (DCNM) and network border gateway switches associated with a multi-site cloud computing network to provide reachability data indicating physical links between the border gateways disposed in different sites of the multi-site network to establish secure connection tunnels utilizing the physical links and unique encryption keys. The SDN controller and/or DCNM may be configured to generate a physical underlay model representing the physical underlay, or network transport capabilities, and/or a logical overlay model representing a logical overlay, or overlay control-plane, of the multi-site network. The SDN controller may also generate an encryption key model representing the associations between the encryption keys and the physical links between the associated network border gateway switches. The SDN controller may utilize the models to determine route paths for transmitting network traffic spanning over different sites of the multi-site network at line speed.