What is Intermediate system to intermediate system (IS-IS)?

What is IS-IS?

IS-IS (Intermediate System to Intermediate System) stands out as a reliable and scalable option for large networks. Originally developed by Digital Equipment Corporation (DEC), IS-IS is now widely used in various network environments.

IS-IS operates at the link-state level, making it similar to OSPF (Open Shortest Path First). However, IS-IS offers some unique features that set it apart from OSPF and make it an attractive choice for many network administrators.

It falls into the category of Interior Gateway Protocols (IGPs) and is primarily employed within Internet Service Providers (ISPs) and large enterprise networks. IS-IS is designed to facilitate efficient routing of data packets between routers and plays a vital role in the functioning of the modern internet.


History and Development

IS-IS dates back to the late 1970s when it was initially designed as part of the OSI protocol suite. Over time, it evolved and adapted to various networking needs, becoming a reliable and scalable protocol.

How does IS-IS work?

IS-IS operates on a hierarchical routing structure, dividing the network into areas. Each area has a designated IS-IS router, known as the Level 1 Intermediate System (L1 IS). These L1 IS routers exchange routing information within their respective areas.

Additionally, there are Level 2 Intermediate System (L2 IS) routers that connect different areas and exchange routing information between them. This hierarchical approach allows for efficient routing and reduces the amount of routing traffic in large networks.

Key components of IS-IS operation include:

  • Hello Protocol: Routers use Hello packets to discover and establish adjacencies with neighboring routers. These adjacencies are crucial for the exchange of routing information.
  • Link-State Packets (LSPs): Routers periodically generate LSPs containing information about their local network segment. LSPs are flooded throughout the network, ensuring that all routers have a consistent view of the network’s topology.
  • Dijkstra’s Algorithm: IS-IS routers use Dijkstra’s algorithm to calculate the shortest path to reach a destination. This algorithm considers the cost associated with each link and determines the optimal path.

IS-IS uses the Dijkstra algorithm to calculate the shortest path between routers. It builds a link-state database by exchanging Link State Protocol Data Units (LSPs) with neighboring routers. Each LSP contains information about the router’s interfaces, neighbors, and the cost of each link.


IS-IS Hierarchical Structure

IS-IS operates in a hierarchical manner, dividing the network into areas. This hierarchical structure helps in managing and scaling large networks effectively. It uses a two-level hierarchy:

  • Level 1 (L1): This level represents routers within an area and is responsible for local routing.
  • Level 2 (L2): This level connects multiple Level 1 areas and is responsible for routing between areas.

SPF Algorithm

IS-IS employs a Shortest Path First (SPF) algorithm to calculate the best path for routing. This algorithm ensures that the routing tables are always up-to-date and optimized, resulting in efficient data transmission.


IS-IS Routing Tables

IS-IS maintains three types of routing tables:

  • Level 1 Routing Table: Contains information about routes within an area.
  • Level 2 Routing Table: Stores routes for inter-area communication.
  • L1/L2 Routing Table: Combines information from both Level 1 and Level 2 for end-to-end routing.


While OSPF and IS-IS are both link-state routing protocols, they differ in several aspects. OSPF is more commonly used in smaller networks, while IS-IS is preferred for larger networks. IS-IS uses a hierarchical structure, while OSPF uses a flat structure.

Another difference is the metric used for calculating path costs. OSPF uses the cost metric, which is typically based on bandwidth, while IS-IS uses the metric of Administrative Distance (AD) and the Delay metric. IS-IS allows for more flexible metric calculations, making it suitable for networks with diverse link characteristics.

Routing Protocol TypeLink-State Routing ProtocolLink-State Routing Protocol
Hierarchical StructureThree-level hierarchy (Backbone Area, Area, Interface)Two-level hierarchy (Level-1 and Level-2)
Metric CalculationCost-based metricConfigurable metric
Convergence SpeedGenerally slower convergenceKnown for fast convergence
ScalabilityWell-suited for smaller networksParticularly suitable for large networks
ComplexityConfiguration can be complex, especially in larger networksInitial setup may be complex, especially for beginners
Resource ConsumptionGenerally consumes fewer resourcesCan be more resource-intensive
Vendor CompatibilityVendor-specific implementations may varyVendor-agnostic, promoting interoperability

Please note that the choice between OSPF and IS-IS depends on the specific requirements and characteristics of your network. Each protocol has its strengths and weaknesses, making it essential to consider your network’s size, complexity, and goals when selecting the appropriate routing protocol.


Advantages of IS-IS

IS-IS offers several advantages that make it a popular choice among network administrators:

  • Scalability: IS-IS is highly scalable and can handle large networks with thousands of routers.
  • Fast Convergence: IS-IS has fast convergence times, allowing for quick adaptation to network changes.
  • Flexibility: IS-IS allows for flexible metric calculations, making it suitable for networks with diverse link characteristics.
  • Security: IS-IS includes built-in mechanisms for authentication and encryption, enhancing network security.

Disadvantages of IS-IS

Despite its many advantages, IS-IS also has some limitations:

  • Complex Configuration: IS-IS can be complex to configure, requiring a good understanding of its hierarchical structure and metric calculations.
  • Less Commonly Used: While IS-IS is widely used in certain network environments, it is less commonly used compared to OSPF.
  • Limited Vendor Support: Some network equipment vendors may have limited support for IS-IS, making it less accessible in certain environments.

IS-IS is a robust routing protocol that offers scalability, fast convergence, and flexibility for large networks. Its hierarchical structure and efficient routing make it an attractive choice for network administrators dealing with complex networks. However, it does require a good understanding of its configuration and may have limited vendor support in some cases. Overall, IS-IS is a powerful option for those looking for a reliable routing protocol in large network environments.

Leave a Reply

Your email address will not be published. Required fields are marked *