The infrastructure layer is composed of networking
equipment and elements that form the actual network
and elements that help to forward network traffic. It
could be a set of network switches and routers in the
network or data centers. The infrastructure layer is the
physical layer over which network virtualization
functions lay via the control layer.
The control layer is where the SDN controllers reside to
control network infrastructure. The control layer has the
business logic to fetch and maintain different types of
network information, state details, topology details, and
statistics details. The SDN controller is all about
managing networks, and it has all the control logic for
network use cases such as switching, routing, Layer 2
VPNs, Layer 3 VPNs, firewall security rules, DNS, DHCP,
and clustering. Cisco implements various services in its
SDN controllers. These services expose APIs (typically
REST-based) to the upper layer (application layer),
which makes life easy for network administrators who
use apps on top of SDN controllers to configure, manage,
and monitor the underlying network. The control layer
exposes two types of interfaces:
Northbound interface: This interface is used for communication
with the upper layer (the application layer) and is in general realized
through REST APIs of SDN controllers.
Southbound interface: This interface is meant for communication
with the lower layer (the infrastructure layer) of network elements and
is in general realized through southbound protocols, such as OpenFlow
and NETCONF.The application layer is a developing area where a lot of
innovations are happening. The applications in this layer
leverage information about the network topology,
network state, network statistics, and so on. Several
types of applications can be developed, such as those
related to network automation, network configuration
and management, network monitoring, network
troubleshooting, network policies, and security. Such
SDN applications can provide various end-to-end