What is IS-IS Protocol and How Does it Work?

IS-IS is a dynamic routing protocol that plays a pivotal role in modern IP networking. It ensures efficient and reliable delivery of data by determining optimal paths within a network.

In this article, we will explain the IS-IS protocol, its features, and how it works. We will also learn about the technical concepts like IS-IS components, types, advantages, use cases, and many more.

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What is IS-IS Protocol?

The IS-IS protocol stands for Intermediate System to Intermediate System. It is a type of Interior Gateway Protocol (IGP) and is used for routing within a single administrative domain, like an organization's or service provider's network.

IS-IS protocol functions by distributing link-state information across the network to ensure reliable and accurate routing. Each router running the IS-IS protocol collects this data to independently build a complete view of the network topology.

Like OSPF, another link-state protocol, IS-IS applies Dijkstra’s algorithm to determine the most efficient route. Once the optimal path is calculated, data packets follow that route to reach their destination.

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History of IS-IS

Originally developed for the OSI model to route CLNP traffic, IS-IS was later adapted for IP, resulting in Integrated IS-IS, a protocol well-suited for modern networks. Unlike OSPF, which runs at the network layer, IS-IS operates directly over the data link layer, offering greater flexibility and efficiency.

Today, IS-IS is widely used in critical infrastructures like Internet backbones, telecom networks, and large enterprises due to its fast convergence, scalable design, and support for extensible features through TLVs.

How Does IS-IS Protocol Work?

The IS-IS protocol functions through a structured series of steps that enable routers to construct a synchronized view of the network and determine the most efficient routes.

It begins with neighbor discovery, where routers send Hello packets to identify and establish relationships with directly connected neighbors, sharing key identifiers such as System ID and Area ID.

Once adjacencies are formed, routers exchange Link-State Packets (LSPs) containing detailed information about interfaces, neighboring routers, and link metrics.

This data is flooded across the network to ensure all routers receive consistent information. Each router then builds a Link-State Database (LSDB), which serves as a complete map of the network's topology.

Using this database, routers run Dijkstra’s Shortest Path First (SPF) algorithm to calculate the most efficient routes to all destinations. Finally, the results of the SPF calculation are used to build a routing table, which guides packet forwarding based on the optimal next-hop paths.

Components of the IS-IS Protocol

IS-IS uses several key elements to uniquely identify routers and structure the routing process:

1. Network Entity Title (NET): A globally unique identifier for each IS-IS router, consisting of an Area ID, System ID, and N-selector (usually set to 00).

2. System ID: A 6-byte unique identifier (often derived from a router’s loopback IP address) that distinguishes each router in the network.

3. Area ID: Identifies the logical IS-IS area a router belongs to. All Level-1 routers in the same area must share this value.

4. Link-State Packet (LSP): Carries routing and topology information. It forms the foundation of the LSDB.

5. Complete Sequence Number Packet (CSNP): Sent by designated routers to summarize known LSPs. It helps neighbors detect missing or outdated information.

IS-IS Routing Tables

IS-IS maintains and synchronizes multiple routing-related tables to ensure optimal path selection:

1. Neighbor Table: Lists adjacent routers with which Hello packets have been successfully exchanged.

2. Topology Table (Link-State Database): Contains all LSPs received from neighbors, creating a complete map of network topology.

3. Routing Table: Derived from SPF calculations, it includes the best paths to all reachable destinations.

Addressing Schemes in IS-IS Protocol

IS-IS relies on ISO CLNS (Connectionless Network Service) addressing, formatted as a Network Entity Title (NET). This unique router identifier includes:

1. AFI (Authority and Format Identifier): Defines address type and length (e.g., 0x49 for private use).

2. IDI (Initial Domain Identifier): Identifies the organization or domain.

3. HO-DSP (High Order Domain Specific Part): Defines the area within the domain.

4. System ID: Typically a MAC or IP address; must be unique within an area.

5. NSEL (NSAP Selector): Indicates the upper-layer protocol, often set to 0 for IS-IS.

How Does IS-IS Protocol Form Adjacencies

The adjacencies in the IS-IS protocol refer to the relationships formed between neighboring routers (called Intermediate Systems) that exchange routing information.

The IS-IS protocol forms adjacencies via IIH packets, which are exchanged every 10 seconds. Adjacencies can be:

Level-1: Within the same area (intra-area)

Level-2: Between different areas (inter-area)

Routers must meet the following requirements to form adjacencies:

● Matching MTU

● Identical area ID (for Level-1)

● Synchronized Hello intervals

● Same authentication settings (if used)

● Same IS level (1, 2, or both)

On broadcast networks, routers elect a DIS (Designated Intermediate System), similar to OSPF’s DR. However, IS-IS DIS elections are preemptive, and there is no backup DIS.

IS-IS Protocol Message Types

When two routers form an adjacency, they use different messages to communicate with each other. Some of the IS-IS messages are:

1. Hello (IIH – IS-IS Hello)

Used to discover and maintain neighbor relationships. Different types of Hello messages exist for point-to-point and broadcast links.

2. Link-State Packet (LSP)

Conveys routing and topology information about the router’s links. They’re flooded throughout the IS-IS domain.

3. Complete Sequence Number Packet (CSNP)

Sent periodically on broadcast networks to advertise all known LSPs. Helps detect missing or outdated LSPs.

4. Partial Sequence Number Packet (PSNP)

Used to acknowledge received LSPs or request specific LSPs when a router detects it is missing some.

Understanding IS-IS Hierarchical Design

IS-IS uses a two-level hierarchy to organize large networks into smaller, manageable segments. This design improves scalability, stability, and routing efficiency, especially in enterprise and service provider environments.

How IS-IS Hierarchy Works?

Instead of multiple area types like OSPF, IS-IS uses router levels to define the scope of routing:

Advantages of IS-IS

The advantages of IS-IS are:

● ISPs and large enterprise networks prefer IS-IS for its ability to handle thousands of routes efficiently.

● Works natively with OSI and has been adapted for IP; some implementations also support IPv6.

● Its topology-driven SPF algorithm ensures rapid convergence and stability.

● Since IS-IS doesn’t rely on IP for its operation, it is less affected by IP-layer misconfigurations.

● Unlike OSPF, which requires a strict backbone area (Area 0), IS-IS has more relaxed area configurations.

Disadvantages of IS-IS

While IS-IS offers powerful features for large-scale routing, it is not without its challenges. Here are some key drawbacks:

● IS-IS is considered more complex than other IGPs like OSPF.

● Limited Vendor Support

● Complex Configuration and Troubleshooting

● Lack of Native IPv6 Support in Early Versions

● Less Visibility in Certification Programs

IS-IS vs. OSPF

Both IS-IS and OSPF are link-state routing protocols, but they work differently. The table below compares the IS-IS and OSPF protocols to know the differences between them.

Use Cases of the IS-IS Protocol

The following are its important use cases:

1. ISPs prefer IS-IS for its fast convergence, scalability, and independence from IP. Tier-1 ISPs, such as AT&T, utilize it for stable, large-scale routing with minimal downtime.

2. Used in MPLS and 5G backbones, IS-IS supports traffic engineering and TLV extensions.

3. IS-IS scales well in spine-leaf architectures and offers efficient flooding. Global financial institutions use it for fast failover across core routers.

4. With Multi-Topology Routing (MTR), IS-IS handles IPv4 and IPv6 separately. Universities adopt IS-IS for smooth IPv6 transitions alongside IPv4.

5. Operating over Layer 2, IS-IS avoids typical IP-based attacks. Defense networks favor it for enhanced routing security.

Conclusion

Though often overshadowed by OSPF in smaller enterprise settings, IS-IS remains a backbone protocol of choice for organizations requiring reliability, scalability, and robust performance.

With its rich feature set and adaptability to both IP and future networking technologies, IS-IS is a testament to the power of well-engineered protocols that stand the test of time.