Hierarchical OSPF scaling

Hierarchical OSPF (OSPF Hierarchical Design) is a way of designing and implementing OSPF networks in a hierarchical structure to optimize network scalability, reduce the routing overhead and the size of the routing table. Hierarchical OSPF is used when the number of routers and links in the network is large, and the single area OSPF implementation may lead to scalability issues, including:

Overloading the backbone area with excessive routing information leading to slow convergence and frequent recalculations.

Limitations on the number of routers that can participate in the same OSPF area, which can restrict the growth of the network.

Increased memory and processing requirements, leading to higher hardware costs.

Hierarchical OSPF addresses these issues by dividing the network into multiple areas based on geographical, functional or administrative boundaries, and grouping routers that share common features and have similar routing requirements into the same area. The areas are connected to a central area called the backbone area, which serves as the transit area for inter-area traffic.

Hierarchical OSPF design provides the following benefits:

Faster convergence: By dividing the network into smaller areas, the size of the LSDB is reduced, and the SPF algorithm is limited to the local area, which speeds up convergence.

Scalability: Hierarchical OSPF supports unlimited growth of the network by allowing routers to be added to the network without requiring changes to the existing routing infrastructure.

Reduced routing overhead: By dividing the network into multiple areas, the amount of routing traffic is reduced, and the size of the routing table is optimized, which improves network performance. Enhanced security: Hierarchical OSPF allows for the use of different authentication mechanisms at different levels of the hierarchy, which enhances network security.