Intermediate System-to-Intermediate System Protocol


The Intermediate System-to-Intermediate System protocol, often abbreviated as IS-IS, is a routing protocol used by network devices to determine the best way to forward data packets. It’s designed for use within a single administrative domain or network, like an internet service provider or a large corporate network. It’s layer-independent and can support both the Internet Protocol (IP) and the Connectionless Network Protocol (CLNP).


The phonetics of the keyword: Intermediate System-to-Intermediate System Protocol is:In-tur-mee-dee-it Sis-tem-toh-In-tur-mee-dee-it Sis-tem Pro-to-kol.

Key Takeaways

  1. IS-IS Protocol is a link-state routing protocol developed by the ISO (International Organization for Standardization) for use in their network layer protocol. It is widely used in large networks due to its scalability and stability.
  2. IS-IS Protocol operates on the basis of areas, where routers within the same area exchange link-state information. This results in a reduction in routing protocol traffic and a simplified network structure.
  3. Unlike other routing protocols like OSPF, IS-IS doesn’t require IP to function. Instead, it uses Network Service Access Points (NSAPs) for addressing. This makes it flexible and applicable to more than just IP networks.


The Intermediate System-to-Intermediate System (IS-IS) Protocol is a crucial concept in technology due to its significant role in routing network packets in many large scale operations. Often used in large telecommunications networks, IS-IS allows routers to determine the best path for datagrams through a packet-switched network. This protocol operates within a network, mapping out the structure and using algorithms to send network traffic on the fastest and most reliable route while automatically adapting to bandwidth usage and network failures. Without IS-IS, efficient and flexible data transmission across large communication networks would be far more challenging. Recognizing its importance is essential to understanding the effective management of data flow and network infrastructure in today’s digital age.


The Intermediate System-to-Intermediate System (IS-IS) Protocol serves an important purpose in network routing. Primarily used within large network systems like an Internet Service Provider (ISP) or a large enterprise network, this protocol is responsible for determining the best path through the network for data to travel. It achieves this by storing information about the network’s layout and using it to calculate the optimal route. This process is particularly vital for the Internet’s overall function, where data typically needs to travel across many different networks to reach its destination.IS-IS Protocol was designed on the basis of the OSI (Open Systems Interconnection) model and operates in Layer 2 – the Data Link Layer. It covers the internal parts of a routing domain and uses a link-state protocol algorithm to find the most effective path for data transmission. The key function of IS-IS is to facilitate speed and efficiency in the data exchange over large networks or over clustered routers. It is also used in IP (Internet Protocol), IPX (Internetwork Packet Exchange), and other protocols that use the Network Layer of the OSI model. Despite having been developed in the 1980s, IS-IS remains a standard routing protocol and its use is still widespread due to its scalability and robustness.


1. Networking in a Large Corporate Office: In large businesses or corporations, different departments or sectors often have their own individual networks. The IS-IS protocol would be used here to route data packets between these subnetworks and the larger network. IS-IS is especially helpful in this context because it’s designed to work with complex, hierarchal networking systems.2. Internet Service Providers: IS-IS protocol is often used by ISPs to route information correctly between different networks. As ISPs have to handle communication for hundreds or even thousands of individual subnetworks, using IS-IS ensures that data is routed correctly and efficiently.3. Telecommunication Networks: Telecom companies make use of IS-IS protocol in their internal network for dynamic routing. The telecom industry has a massive network infrastructure that spans across cities, countries, and even continents. Data needs to be routed quickly and correctly between different networks which can be achieved via IS-IS.

Frequently Asked Questions(FAQ)

Q: What is the Intermediate System-to-Intermediate System (IS-IS) Protocol?A: The IS-IS Protocol is a link-state protocol used for routing within a single administrative domain like a corporate network. It is widely used in large IP networks, primarily in telecommunications.Q: What is the purpose of the IS-IS Protocol?A: The IS-IS Protocol is used to identify the best path for data routing through a packet-switched network based on the state of the links within the network.Q: How does IS-IS Protocol function?A: The IS-IS protocol functions by initially establishing neighbor relationships between routers in a network. It then generates a topographic map of network connections, identifying the fastest routes based on factors such as link speed and capacity. Q: Is IS-IS Protocol limited to IP routing only?A: No, an advantage of IS-IS over some other protocols is that it can be used for routing network layer protocols other than IP, such as the Connectionless Network Service (CLNS). Q: How does IS-IS Protocol handle changes or failures in network links?A: IS-IS protocol implements algorithms to quickly adapt to changes or failures in network links, ensuring data is rerouted through the next most efficient path.Q: Is IS-IS protocol used in small or large networks?A: The IS-IS protocol is typically used in large scale networks because it is highly scalable, mainly in ISP and telecommunications networks.Q: What are the types of IS-IS routers?A: There are three types: Level 1 (L1) routers, Level 2 (L2) routers, and L1-L2 routers. L1 routers only know about the topology in their own area, L2 routers know about all areas in the network, and are considered the backbone, whereas L1-L2 routers act as a bridge between L1 and L2 areas.Q: Can IS-IS protocol and OSPF protocol work together in the same network?A: While IS-IS and OSPF perform similar functions, they use different algorithms and databases and are generally not used together within the same routing domain. However, they can coexist in a network if it’s separated into different routing domains. Q: Is IS-IS protocol secure?A: IS-IS protocol itself does not contain built-in security features. However, network administrators can implement additional measures, such as encrypted tunnels, to enhance the security of networks relying on IS-IS for routing.

Related Tech Terms

  • Routing Protocol
  • Link-State Protocol
  • Open Shortest Path First (OSPF) Protocol
  • Network Topology
  • Internet Protocol (IP)

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