## 3 Related Works
MPLS relies on the agreement between LSRs on how to interpret labels, which is essential for forwarding traffic between and through LSRs. LDP is a set of protocols used for this purpose. Each LSR shares its bindings with other LSRs to ensure consistent label interpretation [12]. In VPLS, a PE acts as an edge router capable of executing LDP signaling and/or routing protocols to establish PWs. Additionally, a PE can create tunnels to communicate with other PEs and forward traffic through PWs. Any modification to the VPLS topology, such as the addition or removal of a PE device, necessitates updating the configuration of every PE within that particular VPLS instance. In a flat VPLS architecture, the control plane is primarily responsible for auto-discovery and the establishment and termination of PWs. However, scalability issues can arise due to the requirement of a complete mesh of PWs between VPLS peers. Additionally, broadcasting messages to every speaker can impair the scalability of VPLS [46]. The hierarchical architecture of VPLS largely resolves scalability challenges. VPLS designs, such as H-VPLS and SD-VPLS, exhibit superior control plane scalability compared to flat systems, such as BGP-based VPLS and LDP-based VPLS. To enhance the scalability of VPLS, the authors of reference [47] proposed a method that enables the utilization of the same tunnels or PWs for multiple clients. In the data plane of VPLS, two key responsibilities are routing packets and encapsulating Ethernet frames. One significant scalability issue in the data plane is the MAC table explosion [12]. For instance, if each PE router is connected to N customer terminals for each service instance and there are M sites in the customer network, the total number of entries stored in each PE is N \(\times\) M. When dealing with a large number of terminals (e.g., 25,000) and multiple locations (e.g., 10), each PE router would need to handle 250,000 entries. This quantity is typically beyond the capacity of most ethernet switches [48]. Several studies, including references [44; 49], have investigated and compared the scalability of different VPLS implementations. Additionally, reference [50] proposed a scalable virtual Layer 2 implementation, and reference [6] suggested various methods to enhance the scalability of VPLS. Each of these studies is discussed below. In their work, the authors of reference [49] conducted a comparative analysis of the control plane scalability of L2 VPN and L3 VPN based on MPLS. This research examined and compared the performance of L2 VPN and L3 VPN using various metrics, including creation and deletion times, control plane memory usage, and overall memory usage. The findings of this study serve as a foundation for future scalability comparisons between traditional VPNs and their Software-Defined Networking (SDN)-based counterparts. Reference [50] proposed a scalable virtual Layer 2 implementation. Their study focused on analyzing the scalability of virtual Layer 2 for applications that utilize server clusters, such as data centers, as well as globally distributed applications. The research explored both hardware and software factors that can potentially impact VPLS compatibility. Given that VPLS deployments often involve equipment from different suppliers, compatibility issues may arise. To address this challenge, reducing reliance on vendor-specific hardware is crucial. These studies contribute to the understanding of VPLS scalability and propose solutions to address compatibility and performance issues. Future research in this area can build upon these findings to further enhance the scalability of VPLS deployments and explore the potential benefits of SDN-based approaches. In general, the current H-VPLS architectures often involve complex network management. However, the use of the Q-in-Q technique presents an opportunity to extend VLANs across a service provider's network by adding an extra VLAN tag to the primary ethernet frame.