CORD expects two networks: a management network (for control traffic between the control plane containers) and a dataplane network (for end-user traffic).
The management network that connects all physical devices (compute machines, fabric switches, access devices, development machines), allowing them to communicate to manage CORD. This is usually a 1G copper network, but may vary deployment by deployment. Network devices (access devices and fabric switches) usually connect to this network through a dedicated management 1G port. If everything is setup correctly, any device should be able to communicate with the others at L3 (i.e., devices should ping one each other).
The management network is usually used to access Internet for the underlay infrastructure setup, although CORD doesn't require Internet access. For example, you will likely need to have Internet access through this network to install or update your OS, switch software, Kubernetes, and so on.
The following is a diagram of a typical management network.
The dataplane network carries the users' traffic, connecting subscribers to the Internet (which is the whole purpose of CORD). The following is a diagram of a reference dataplane network for CORD.