Design and Implementation of a Resilient Multi-Branch WAN for Sonali Bank PLC: EIGRP Routing, Dual-ISP Redundancy, and Hierarchical IP Addressing

Modern banking operations depend on a wide-area network (WAN) that is simultaneously highly available, secure, and capable of scaling to thousands of geographically dispersed branches. This paper presents the design and packet-level implementation of a resilient enterprise WAN interconnecting a Head Office (HO), a primary Data Center (DC), a Near/Disaster-recovery Data Center (NDC), and re-
mote branches through a mesh of five Internet Service Provider (ISP) edge routers. The control plane uses the Enhanced Interior Gateway Routing Protocol (EIGRP, AS 100) for fast con-vergence and unequal-cost load balancing over redundant /30
links, and every site is dual-homed to two independent ISP
routers to eliminate single points of failure. Security is enforced at each data-center edge with Cisco Adaptive Security Appliance (ASA) firewalls using asymmetric security levels and stateful inspection. A hierarchical IPv4 plan of contiguous /21 blocks from 10.0.0.0/8 yields more than 1,230 branch subnets of 2,046 usable hosts each while preserving route summarization. Emulation in Cisco Packet Tracer confirms full reachability, sub-second recon-vergence after link failure, and correct fire- wall policy enforcement, demonstrating that the architecture meets the availability, segmentation, and scalability requirements of a national-scale banking network.