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Definition of IS-IS

Intermediate System to Intermediate System (IS-IS) is a routing protocol used in computer networks to efficiently exchange routing information between routers. It falls under the link-state routing protocol category, meaning it disseminates information about network topology to all routers in the network. IS-IS operates at the OSI layer 2 (Data Link Layer) and layer 3 (Network Layer).

Origin of IS-IS

Originally developed by Digital Equipment Corporation (DEC) in the 1980s, IS-IS was standardized by the International Organization for Standardization (ISO) in 1990. It was designed to work within the OSI protocol suite, which was an alternative to the TCP/IP protocol suite that gained more widespread adoption. Despite the decline of the OSI model in favor of TCP/IP, IS-IS found its niche in certain networks, particularly Internet Service Provider (ISP) backbones and large enterprise networks.

Practical Application of IS-IS

One practical application of IS-IS is in large-scale networks where scalability and efficiency are paramount. It is commonly used in ISP backbones and telecommunications networks due to its ability to handle large routing tables and adapt to changes in network topology quickly. Additionally, IS-IS is used in some data center environments where high availability and rapid convergence are crucial.

Benefits of IS-IS

Scalability: IS-IS is highly scalable, making it suitable for large networks with thousands of routers. Its hierarchical design allows for efficient routing information exchange without causing excessive overhead.

Fast Convergence: IS-IS converges quickly in response to network topology changes, ensuring minimal disruption to network traffic. This rapid convergence is critical in networks where downtime is costly.

Flexibility: IS-IS supports multiple network topologies, including broadcast, point-to-point, and non-broadcast multi-access (NBMA). This flexibility allows it to adapt to diverse network environments.

Traffic Engineering: IS-IS provides mechanisms for traffic engineering, enabling network administrators to control the flow of traffic through the network efficiently.


IS-IS and OSPF are both link-state routing protocols, but they differ in their underlying protocols and some operational aspects. IS-IS operates at the OSI layer 2 and 3, while OSPF operates at the IP layer. IS-IS uses a different metric calculation method compared to OSPF, and IS-IS has a more flexible addressing scheme.

While IS-IS is commonly associated with large-scale networks, it can also be used in smaller networks. However, its complexity and overhead may outweigh the benefits in very small networks where simpler routing protocols like RIP or static routing suffice.

IS-IS itself does not provide built-in security mechanisms like authentication or encryption. However, it can be used in conjunction with other security measures such as IPsec or Virtual Private Networks (VPNs) to secure routing information exchange between routers.


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