Exploring Link-State Routing Protocols: OSPF and IS-IS

Exploring Link-State Routing Protocols: OSPF and IS-IS

Link-state routing protocols represent a significant evolution in network routing, offering a more efficient and dynamic approach compared to distance vector protocols. By maintaining a complete map of the network topology, link-state protocols enable routers to make more informed and optimal routing decisions. Let’s delve into the intricacies of link-state routing, with a particular focus on OSPF (Open Shortest Path First) and IS-IS (Intermediate System to Intermediate System).

What Are Link-State Routing Protocols?

Link-state routing protocols operate on the principle that each router has an identical map of the network topology. This map is built by routers advertising their connectivity information to all other routers in the network until a consistent view is established across the network.

Key Concepts:

  • Network Topology Map: Each router maintains a complete and identical map of the network.
  • Link-State Advertisements (LSAs): Routers broadcast LSAs to share their local connectivity information with all other routers.
  • Shortest Path First (SPF) Algorithm: Each router independently calculates the best routes using the SPF algorithm (Dijkstra’s algorithm).

Difference from Distance Vector Protocols

  • Knowledge of Network: Link-state routers know the entire network topology, while distance vector routers only know their neighbors and rely on them for further routing information.
  • Update Mechanism: Link-state protocols flood LSAs to the entire network, whereas distance vector protocols send updates only to immediate neighbors.
  • Resource Usage: Link-state protocols use more memory and processing power to maintain the topology map and calculate routes.
  • Reaction to Changes: Link-state protocols react faster to network changes due to their comprehensive view of the network, allowing quicker recalculations of the best paths.

OSPF (Open Shortest Path First)

OSPF is a widely used link-state routing protocol designed for IP networks. It is known for its efficiency and scalability, making it suitable for large and complex networks.

Key Features:

  • Hierarchical Design: OSPF uses a two-level hierarchy with areas to optimize routing and reduce overhead. The backbone area (Area 0) connects all other areas.
  • Router Types:
    • Internal Router: Resides within a single area.
    • Backbone Router: Part of the backbone area.
    • Area Border Router (ABR): Connects different areas.
    • Autonomous System Boundary Router (ASBR): Connects to external networks.
  • Metric: OSPF uses a cost metric, which is typically based on the bandwidth of the links. The lower the cost, the more preferred the route.

OSPF Operation:

  1. Neighbor Discovery: Routers discover their neighbors on directly connected networks.
  2. Database Synchronization: Routers exchange LSAs to synchronize their link-state databases.
  3. SPF Calculation: Each router independently runs the SPF algorithm to calculate the shortest path to all destinations.
  4. Route Installation: The best routes are installed in the routing table.

Determining the Best Route:

OSPF calculates the best route based on the cumulative cost of reaching a destination. The cost is derived from the link bandwidth:

  • Higher Bandwidth = Lower Cost
  • Lower Cost = Preferred Route

IS-IS (Intermediate System to Intermediate System)

IS-IS is another link-state routing protocol, primarily used in large service provider networks. It operates similarly to OSPF but has some distinct features.

Key Features:

  • Flexible Hierarchical Design: IS-IS supports multi-level hierarchy without a strict backbone area.
  • Network Layer Protocol Independence: Initially designed for CLNS, IS-IS can route both IP and non-IP traffic.
  • Link-State PDUs (LSPs): Equivalent to OSPF’s LSAs, LSPs contain link-state information.

IS-IS Operation:

  • Level-1 and Level-2 Routing: Similar to OSPF’s intra-area and inter-area routing.
  • SPF Calculation: Each router uses the SPF algorithm to determine the shortest path.

Equal-Cost Multi-Path (ECMP)

ECMP allows multiple paths with the same cost to be used for routing packets to the same destination. This enhances load balancing and redundancy.

How ECMP Works:

  • Dynamic Routing: In dynamic routing protocols like OSPF, if multiple routes to the same destination have the same cost, all are installed in the routing table.
  • Static Routing: Static routes can also use ECMP if multiple static routes to the same destination are configured with the same administrative distance.

Advantages of ECMP:

  • Load Balancing: Distributes traffic across multiple paths, improving bandwidth utilization.
  • Redundancy: Provides alternative paths in case of link failures.

Administrative Distances

Administrative distance (AD) is a value that ranks the trustworthiness of different routing protocols. Lower AD values are preferred over higher ones.

Common Administrative Distances:

ProtocolAdministrative Distance
Static Route1

Comparing Metrics Across Protocols

Each routing protocol uses different metrics to determine the best path:

  • RIP: Hop count.
  • EIGRP: Composite metric based on bandwidth and delay (by default).
  • OSPF: Cost, typically based on link bandwidth.
  • IS-IS: Cost, similar to OSPF.

Static Routes and ECMP:

Static routes do not inherently use metrics for path selection. Instead, they rely on administrative distance. However, static routes can support ECMP by configuring multiple static routes with the same AD to achieve load balancing and redundancy.

Advantages and Disadvantages of Link-State Protocols


  • Fast Convergence: Quick adaptation to network changes due to the complete topology view.
  • Scalability: Suitable for large and complex networks with hierarchical design.
  • Optimal Routing: Precise calculation of the best routes based on comprehensive network information.


  • Resource Intensive: Requires more memory and CPU to maintain the topology database and perform SPF calculations.
  • Complex Configuration: More complex to configure and manage compared to simpler distance vector protocols.


Link-state routing protocols like OSPF and IS-IS provide robust and efficient routing solutions for large and dynamic networks. By maintaining a complete map of the network topology, they offer fast convergence, optimal routing, and scalability. Understanding the operation, metrics, and administrative distances of these protocols is crucial for network engineers to design and manage efficient and resilient networks.

Whether to use OSPF, IS-IS, or another protocol depends on the specific requirements and scale of the network. OSPF’s hierarchical design and widespread support make it ideal for large enterprise networks, while IS-IS’s flexibility and protocol independence suit large service provider environments.

Share this post