What is Intermediate System to Intermediate System (IS-IS)?

Learn what Intermediate System to Intermediate System (IS-IS) is and how it operates. Discover its significance, functionalities, and intricacies explained in detail.

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Dimitri Nek
Dimitri Nek
Web Hosting Geek


In networking, one protocol that often piques the interest of network enthusiasts is the Intermediate System to Intermediate System (IS-IS). But why is it so important?

As you dig into this article, you’ll uncover the intricacies of IS-IS, its significance in modern networking, and how it stands out from other routing protocols. By the end, you’ll have a comprehensive understanding of IS-IS and its pivotal role in ensuring seamless network communication.

Let’s get started.

What is Intermediate System to Intermediate System?

IS-IS, or Intermediate System to Intermediate System, stands as a pivotal link-state routing protocol, predominantly employed within expansive networks, notably among Internet Service Providers.

Intermediate System to Intermediate System diagram

Tracing its roots back to the OSI (Open Systems Interconnection) model, IS-IS is meticulously crafted to facilitate the swift and efficient transmission of information within intricate computer networks. This ensures that data packets traverse the optimal routes, minimizing potential bottlenecks or inefficiencies.

A distinctive feature of IS-IS is its operation at the data link layer of the network layer, as opposed to many other protocols that operate at the IP level. This unique operational level grants IS-IS the flexibility to manage a diverse range of data, transcending beyond mere IP datagrams.

Its adaptability and robustness make it an invaluable asset in the realm of network routing, especially when dealing with multifaceted data streams and topologies.

How Does IS-IS Function?

Intermediate System to Intermediate System (IS-IS) operates using a systematic and structured approach to ensure seamless data transmission across networks.

IS-IS functioning

The process begins with the formulation of a topological database, which serves as a comprehensive blueprint of the entire network. This database meticulously illustrates the intricate interconnections between various nodes, commonly referred to as routers.

To keep this topological database updated and accurate, routers actively exchange link-state packets (LSPs). These LSPs are essentially data packets containing vital information about the state of links within the network. As routers disseminate and receive these packets, they continually refresh and synchronize their individual views of the network topology, ensuring a unified and consistent understanding across all routers.

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Upon the completion and synchronization of this topological database, the Shortest Path First algorithm comes into play. This algorithm delves into the database to compute the most efficient route for data packet transmission. By identifying the shortest and least congested paths, the SPF algorithm ensures that data is routed with minimal latency, thereby optimizing transmission speed and network performance.

To draw a parallel, consider the intricate workings of shared hosting, where multiple websites share a single server’s resources. Just as IS-IS ensures optimal data routing in a network, shared hosting optimizes resource allocation among websites. For a more detailed exploration of the mechanics behind shared hosting, you might find this article enlightening.

Why Opt for IS-IS Over Alternative Routing Protocols?

When navigating the intricate world of network routing, IS-IS emerges as a standout choice, offering a plethora of benefits that set it apart from other routing protocols.

Here’s a deeper dive into its unique advantages:

  • Rapid Convergence: As a link-state protocol, IS-IS has the innate ability to achieve faster convergence times. This means that in the event of network changes or disruptions, IS-IS can quickly recompute paths, ensuring minimal downtime and maintaining network stability. In contrast, distance-vector protocols often require more time to propagate updates and stabilize.
  • Scalability: With the surge in network complexities and the ever-growing demand for seamless connectivity, scalability becomes paramount. IS-IS, with its robust architecture, can effortlessly accommodate large-scale networks, making it an ideal choice for expansive environments like mega data centers or vast ISP networks.
  • Multi-Protocol Support: Unlike many routing protocols that are tethered to IP-based operations, IS-IS exhibits versatility by functioning directly on the data link layer. This operational depth empowers it to support a myriad of network protocols, transcending the limitations of IP-centric protocols. Whether it’s IPv4, IPv6, or even non-IP protocols, IS-IS can handle them with finesse.
  • Future-Proofing Networks: The digital landscape is in a constant state of flux, with network protocols and topologies evolving rapidly. IS-IS’s multi-protocol capability ensures that it remains not only relevant but also adaptable to these changing dynamics. This adaptability ensures that networks employing IS-IS are better poised to integrate future technological advancements without necessitating massive overhauls.
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For those seeking a routing protocol that melds speed, scalability, versatility, and future-readiness, IS-IS stands as a compelling choice, outshining many of its contemporaries in the networking domain.

Where is IS-IS Commonly Used?

IS-IS, with its robust architecture, is a staple in expansive network environments. Notably, ISPs and sprawling enterprise networks rely heavily on its capabilities. The reason? Its inherent scalability. IS-IS can effortlessly accommodate the growth of a network, ensuring seamless communication even as more nodes or routers are added.

Moreover, the efficiency of IS-IS is unparalleled when it comes to rapid information dissemination. In scenarios where data needs to be broadcasted across a multitude of routers swiftly, IS-IS stands out as the go-to protocol.

Diving deeper into its lineage, IS-IS owes its inception to the OSI (Open Systems Interconnection) model. This association empowers it with a unique versatility. Beyond the realm of standard IP-based networks, IS-IS is adept at handling non-IP protocols. This makes it an invaluable asset in specialized environments that leverage alternative communication protocols, further broadening its application spectrum.

When Was IS-IS Introduced?

IS-IS made its debut in the dynamic world of networking during the 1980s. This introduction was a strategic move under the umbrella of the OSI (Open Systems Interconnection) initiative. At that time, the Routing Information Protocol (RIP) was reigning supreme in the networking domain. IS-IS was envisioned as a formidable alternative to RIP, aiming to address some of its limitations and offer more advanced features.

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As the digital era progressed, the demands and complexities of networking evolved. Recognizing this, IS-IS wasn’t left static. It underwent a series of refinements and enhancements, each iteration making it more adept at handling the intricacies of modern networks. These continuous improvements, coupled with its inherent robust design, have fortified IS-IS’s reputation. Today, it stands tall as a resilient and dependable routing protocol, trusted by network professionals globally.


Intermediate System to Intermediate System (IS-IS) is more than just a routing protocol. It’s a testament to the evolution of networking, offering a blend of efficiency, versatility, and scalability.

Understanding IS-IS is pivotal for anyone keen on diving deep into the world of networking. Its unique features and capabilities set it apart, making it a preferred choice for large-scale network environments. As you venture further into networking, harnessing the power of IS-IS will undoubtedly prove beneficial.

I invite you to share your insights and questions in the comments below.


  1. What makes IS-IS different from other routing protocols?

    IS-IS operates directly on the data link layer, allowing it to handle various data types, not just IP datagrams. Its origin from the OSI model also enables it to support non-IP protocols, making it versatile and adaptable.

  2. Is IS-IS suitable for small networks?

    While IS-IS can function in small networks, it’s primarily designed for large-scale environments like ISPs or extensive enterprise networks due to its scalability and efficiency.

  3. How does IS-IS ensure efficient data routing?

    IS-IS creates a topological database that maps the network. Using the Shortest Path First (SPF) algorithm, it calculates the best path for data transmission, ensuring minimal latency and optimal speed.

  4. When was IS-IS first introduced?

    IS-IS was introduced in the 1980s as part of the OSI initiative to offer an alternative to the Routing Information Protocol (RIP).

  5. Can IS-IS support multiple network protocols?

    Yes, IS-IS’s ability to operate directly on the data link layer allows it to support multiple network protocols, not just IP, making it multi-protocol in nature.


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