## 6 Methodology and Performance Metrics Measurement
### When Link Failure Occurs in EIGRP  
When a network link failure occurs on the primary connection, such as Tunnel-1 from the overlay network established by ISP-A for data traffic, EIGRP quickly detects the issue using its rapid convergence mechanisms. EIGRP then reroutes the data traffic to the alternative connection, Tunnel-2, which is established by ISP-B. EIGRP routers are equipped with various timers and protocols that enable them to promptly identify network changes. Upon detecting a fault, the affected router initiates the Diffusing Update  
Figure 8: Traceroute from Branch-2.  
Figure 7: Traceroute from Branch-1.  
Algorithm (DUAL), which recalculates the routing table to find an alternate path. This transition is facilitated by the exchange of routing information among routers, ensuring that all routers within the EIGRP domain are updated on the changes in network topology. Additionally, precomputed backup routes, or viable successor routes, further reduce convergence time.  
For Internet traffic, when ISP-B is the primary connection and ISP-A is the backup, EIGRP manages a link failure on the primary connection by redirecting traffic to the secondary connection from the underlay network. This seamless failover between primary and secondary connections ensures continuous connectivity and minimizes interruptions during link failures. Figure 9 illustrates the link failure through ISP-A while EIGRP is operational in the network topology.  
Traceroute results for data and Internet traffic from Branch-1 with EIGRP enabled are shown in Figure 10. When a link failure occurs between Branch-1 and ISP-A, the system smoothly transitions data traffic to the backup connection, reducing network disruptions and ensuring uninterrupted data connectivity. Consequently, if the primary tunnel (Tunnel-1) via ISP-A fails, data traffic quickly shifts to the secondary tunnel (Tunnel-2) via ISP-B. Internet traffic, which is already routed through ISP-B, continues smoothly. This setup enables ISP-B to efficiently manage both data and Internet traffic, maintaining a stable network flow.  
Figure 10: Traceroute from Branch-1.  
Figure 9: ISP-A failure.  
If the primary Internet link connecting Branch-1 to ISP-B fails, as shown in Figure 11, the secondary link, which connects Branch-1 to ISP-A, takes over to maintain Internet connectivity from the underlay network. This failover mechanism ensures that Internet traffic is seamlessly redirected through ISP-A, preserving network operation during a primary link failure. Thus, both data and Internet traffic are routed via ISP-A, ensuring minimal downtime for Internet services. For Branch-2, data traffic continues through Tunnel-1 via ISP-A, and Internet traffic is routed through ISP-B. This configuration enhances network resilience, ensuring minimal downtime in the event of a link failure.  
Traceroute findings for data and Internet traffic from Branch-1, when ISP-B fails, are shown in Figure 12. The failover mechanism activates, efficiently redirecting Internet traffic to the backup link, ISP-A, to avoid network disturbances and maintain uninterrupted connectivity. As a result, if the primary connection for Internet traffic via ISP-B becomes unavailable, traffic is automatically rerouted through ISP-A. Data traffic, already routed through ISP-A, ensures that both data and Internet traffic are effectively managed, maintaining network stability.  
A similar failover scenario occurs at Branch-2, where ISP-A is the primary link for data traffic and ISP-B is the backup, while ISP-B is the primary link for Internet traffic and ISP-A is the backup. In the event of a link failure, EIGRP's failover mechanism automatically shifts traffic to the secondary link, ensuring continuous network connectivity.  
Figure 11: ISP-B failure.  
Figure 12: Traceroute from Branch-1.  
This transition is crucial for maintaining stable and uninterrupted network services for businesses and organizations.