## 2 Background and Related Work  
The cost of new technology implementation is generally higher in terms of capital and operational expenditure (CapEX and OpEX) investment and the development of technical human resources (HR) to manage and operate those newer technologies for ISPs and Telcos service providers. There are also certain implementation challenges with respect to management, availability of technological standards and user interface provisioning when providing new technology-based services to customers during and after network migration [7; 8]. A legacy network is less flexible to customized programming and is more vendor specific, leading to higher dependency on support, management and operation. A better solution would be the implementation of SDN. In SDN, networks are controlled by software applications running on the top of an SDN controller. The separation of control and data plane operations in SDN simply transforms the network switches into simple forwarding devices. NOS provides the essential resources and abstractions to facilitate the programming of forwarding devices based on a logically centralized, abstract network view. The resulting network is also programmable through software applications running on top of the NOS, which interact with the underlying data plane devices. ONOS [9] is the robust and distributed controller OS, which provides a better solution for building next-generation SDN/NFV solutions. The need for ISPs and Telcos service providers can simply be fulfilled by the introduction of ONOS to carrier-grade solutions that leverage the economics of white box merchant silicon hardware while offering the flexibility to create and deploy new dynamic network services with simplified programmatic interfaces [9]. ONOS is developed with a set of several other applications that make it flexible, modular, and scalable in terms of both the architecture and the cluster, and the distributed SDN controller. SDN-IP is one of the ONOS applications developed to implement routing with external legacy networks using the standard border gateway protocol (BGP) to enable interoperability with legacy routing. In ONOS/SDN-IP, the SDN can be treated as a single autonomous system (AS) and communicates simply with external AS, as with traditional routing. SDN-IP integrated BGP and ONOS enable communication with external AS in the hybrid SDN so that SDN-IP behaves like a regular BGP speaker and uses its services to install and update the appropriate forwarding state in the SDN data plane devices. [PERSON] et al. [3; 10; 11; 12] presented approaches for legacy network transformation to SDN and IPv6 networking so that service providers can smoothly plan for network transformation with optimised migration cost. The authors implemented IPv4/IPv6 routing in a multi-domain SoDIP6 network using ONOS/SDN-IP. [PERSON] [13] presented the use of multiple ONOS controllers in a cluster for high available services using ONOS/SDN-IP reactive routing. Efficient routing implementations in the different aspects of wired and wireless SDN [14; 15; 16; 17] are the primary concerns, but their implementations in production level networks have to be evaluated considering the smooth transition from legacy to hybrid SDN to pure SDN. There are many studies on hybrid SDN implementations. For example, HARM-LESS [18], Panopticon [19], RouteFlow [20], Fibbing [21] and OSHI [22] are some of the attempts, but most of this research has not been implemented at the production level. OSHI considers the hybrid switch, while our implementation only considers the hybrid SDN/legacy network as a mix network and routs implementations on that mixed network. [PERSON] et al. [23] presented a modified version of three dynamic routing algorithms viz. MIRA, DORA and SWP, to be implemented with the OpenFlow controller. These routing modules are implementable only in an SDN environment.