## 6 Methodology and Performance Metrics Measurement
### When Link Failure Occurs in OSPF  
As a link-state routing protocol, OSPF continually exchanges routing information between routers to maintain an up-to-date map of the network. In this setup, OSPF ensures that data traffic primarily uses Tunnel-1 via ISP-A when it is operational, as it is designated the primary connection. If Tunnel-1 fails, OSPF dynamically reroutes data traffic to Tunnel-2 via ISP-B, enhancing network resilience. For Internet traffic, when ISP-B is the primary link and ISP-A is the secondary link, OSPF adjusts the routing based on real-time network conditions and link statuses. This dynamic adaptability is a key advantage of OSPF, improving network performance and ensuring optimal use of available resources. Figure 13 illustrates the link failure through ISP-A while OSPF is functioning.  
Traceroute results for data and Internet traffic from Branch-2 are shown in Figure 14. When connectivity between Branch-2 and ISP-A fails, the system smoothly reroutes data traffic to the backup link, avoiding network disturbances and ensuring continuous data connectivity. If Tunnel-1 via ISP-A fails, data traffic automatically shifts to Tunnel-2 via ISP-B, while Internet traffic continues through ISP-B from the underlay network. Consequently, both data and Internet traffic are routed through the backup link via ISP-B, maintaining reliable network performance.  
Figure 14: Traceroute from Branch-2.  
Figure 13: ISP-A failure.  
When the primary Internet connection between Branch-2 and ISP-B fails, the network quickly reroutes Internet traffic to the secondary connection through ISP-A. This automatic transition allows for a swift response to connection failures, ensuring uninterrupted Internet access despite the outage of the primary link. This failover strategy enhances network reliability and stability by dynamically adapting to changing conditions and maintaining connectivity for critical services. For Branch-1, data traffic uses Tunnel-1 via ISP-A and Internet traffic is routed through ISP-B from the underlay network. Figure 15 shows the link failure through ISP-B with OSPF in operation.  
Traceroute results for data and Internet traffic from Branch-2 are shown in Figure 16. When the primary Internet connection through ISP-B fails, the system automatically reroutes Internet traffic through the secondary connection via ISP-A. Data traffic continues through Tunnel-1 via ISP-A. This configuration ensures that both data and Internet traffic are routed through ISP-A, effectively mitigating the impact of the primary connection failure and supporting a stable network design.  
A similar scenario applies to Branch-1, where ISP-A is the primary link for data traffic from the overlay network and ISP-B is the secondary link. Conversely, ISP-B is the primary link for Internet traffic from the underlay network, and ISP-A is the secondary link. In the event of a link failure, OSPF's failover mechanism automatically shifts traffic to the secondary link, reducing downtime and maintaining continuous communication. This  
Figure 16: Traceroute from Branch-2.  
Figure 15: ISP-B failure.  
redundancy approach enhances network resilience, minimizes the impact of connection failures, and ensures efficient traffic rerouting, thereby improving overall network stability and efficiency.