Static routing requires manual configuration by network administrators, necessitating adjustments when network changes occur, limiting its large-scale application in modern networks.

Dynamic routing protocols are widely adopted due to they flexibility, reliability, and scalability. Among these, OSPF (Open Shortest Path First) is one of the most extansively used dynamic routing protocols.

Defined in RFC 2328, OSPF is a link-state routing protocol based on the Shortest Path First algorithm.

Role of Dynamic Routing

Static routes are manually configured and maintained by engineers, suitable for small or stable networks. Key limitations include:

• Poor scalability: Configuration complexity grows exponentially with network size.
• Lack of dynamic adaptation: Manual intervention required for network changes.

Classification of Dynamic Routing Protocols

Distance Vector Protocols

Routers periodically flood their routing tables. Each router learns routes from neighbors and updates its own table without understanding the full network topology.

Link-State Protocols - LSA Flooding

Unlike distance vector protocols, link-state protocols exchange Link State Advertisements (LSAs) after establishing neighbor relationships.

Link-State Protocols - LSDB Formation

Each router generates LSAs and stores received LSAs in its Link State Database (LSDB), building a complete network topology map.

Link-State Protocols - SPF Calculation

Routers use the Shortest Path First algorithm on the LSDB to compute loop-free shortest-path trees rooted at themselves.

Linnk-State Protocols - Routing Table Generation

Optimal paths are loaded into the routing table.

OSPF Overview

OSPF is a quintessential link-state protocol supporting IPv4 (OSPFv2, RFC 2328) and IPv6 (OSPFv3, RFC 2740). Key characteristics:

• Routers exchange LSAs rather than routes directly.
• LSDB stores complete topology information for loop-free path calculation.
• Supports Variable Length Subnet Masking (VLSM) and manual route summarization.
• Multi-area design enables large-scale deployments.

Key OSPF Concepts

Areas

Logical divisions of networks identified by Area IDs.

Router-ID

A unique identifier for each router, configurable manually or automatically.

Metric Calculation

OSPF uses Cost as its metric. Default interface Cost = 100 Mbps / bandwidth (configurable reference value).
Total path Cost sums all inbound interface Costs along the path.

OSPF Packet Types