Mastering IS-IS: A Comprehensive Guide with Interview Questions

In the ever-evolving landscape of computer networking, the Intermediate System to Intermediate System (IS-IS) routing protocol has emerged as a powerful solution, particularly in service provider networks. Developed for both IP and non-IP environments, IS-IS offers a robust and scalable approach to routing, making it a crucial component of modern network infrastructures.

This article aims to provide a comprehensive understanding of IS-IS, its key features, and its implementation. Additionally, we’ll explore a curated list of interview questions related to IS-IS, helping you prepare for your next technical interview. Whether you’re a seasoned networking professional or an aspiring network engineer, this guide will equip you with the knowledge and insights needed to navigate this essential protocol confidently.

Understanding IS-IS

IS-IS is a link-state routing protocol that operates at the data link layer, making it independent of the underlying network protocol. It was originally designed for the Connectionless Network Service (CLNS) protocol, but later adapted to support IP routing as well. One of the key advantages of IS-IS is its hierarchical architecture, which allows for efficient scaling and management of large networks.

The IS-IS routing protocol is based on the concept of “areas” or “levels,” similar to the hierarchical design of OSPF. There are two levels in IS-IS:

  1. Level 1 (L1): This level encompasses routing within a single area or domain. L1 routers maintain a complete topology of their area but have no knowledge of other areas.

  2. Level 2 (L2): This level handles inter-area routing, connecting multiple Level 1 areas. L2 routers maintain a complete topology of the entire network, including all Level 1 areas.

IS-IS routers can operate at either Level 1, Level 2, or both levels simultaneously (L1/L2 routers). This hierarchical design allows for efficient routing and scalability, as well as the ability to summarize routes between areas, reducing the overall routing table size.

Key Features of IS-IS

IS-IS offers several key features that contribute to its robustness and efficiency:

  1. Link-State Database (LSDB): IS-IS routers maintain a Link-State Database, which contains information about the network topology and the available routes. This database is built and maintained through the exchange of Link-State Protocol Data Units (LSPs).

  2. Designated Intermediate System (DIS): On multi-access networks, such as Ethernet, IS-IS elects a Designated Intermediate System (DIS), which is responsible for creating and flooding LSPs on behalf of the network segment. This reduces the overhead of flooding and ensures efficient propagation of routing information.

  3. Hierarchical Routing: As mentioned earlier, the hierarchical design of IS-IS allows for efficient scaling and management of large networks. Routes can be summarized between areas, reducing the overall routing table size and improving convergence times.

  4. Support for IPv4 and IPv6: IS-IS can handle both IPv4 and IPv6 routing, making it a versatile protocol for modern networks that require support for various IP versions.

  5. Multi-Topology Routing: IS-IS supports Multi-Topology Routing, which allows for the creation of separate topologies for different types of traffic or protocols within the same network. This feature is particularly useful in environments where different routing requirements exist for different applications or services.

  6. Loop-Free Routing: Like other link-state routing protocols, IS-IS ensures loop-free routing by maintaining a consistent view of the network topology across all routers.

Common IS-IS Interview Questions

Now that we’ve covered the fundamentals of IS-IS, let’s explore some commonly asked interview questions on this topic:

  1. What is the purpose of the “attach-bit” in IS-IS?
    The attach-bit is used to indicate that a router is attached to another area or domain. It is set by L1/L2 routers to inform L1 routers in their area that they have connectivity to other areas.

  2. Explain the format of an NSAP (Network Service Access Point) address in IS-IS.
    An NSAP address in IS-IS consists of several fields, including the Authority and Format Identifier (AFI), Initial Domain Part (IDP), Domain-Specific Part (DSP), and Network Service Selector (NSEL). The NSAP address uniquely identifies a node in the IS-IS network.

  3. What are CSNP (Complete Sequence Number PDU) packets, and what is their purpose?
    CSNP packets are used to maintain synchronization of the Link-State Database among IS-IS routers. They contain a complete listing of all LSPs in the LSDB and are periodically transmitted by the Designated Intermediate System (DIS) on multi-access networks.

  4. What are PSNP (Partial Sequence Number PDU) packets, and what is their purpose?
    PSNP packets are used to acknowledge the receipt of LSPs and request missing LSPs from neighboring routers. They contain a partial listing of the LSPs in the LSDB and are sent by non-DIS routers on multi-access networks.

  5. Can an L1 router form an adjacency with an L2 router?
    No, an L1 router cannot form an adjacency with an L2 router directly. L1 routers can only establish adjacencies with other L1 routers or L1/L2 routers within the same area.

  6. How many adjacencies are formed if two L1/L2 routers are connected to each other?
    When two L1/L2 routers are connected, three adjacencies are formed: one L1 adjacency and two L2 adjacencies (one in each direction).

  7. What is the purpose of the “overload-bit” in IS-IS?
    The overload-bit is used to indicate that a router is experiencing an overload condition and should not be used for transit traffic. When the overload-bit is set, other routers will avoid using the overloaded router as a transit router, preventing potential blackholing of traffic.

  8. How does IS-IS handle route summarization?
    IS-IS supports route summarization at both the L1 and L2 levels. L1 routers can summarize routes within their area, while L2 routers can summarize routes between areas. This helps reduce the size of routing tables and improve convergence times.

  9. What is the purpose of “mesh groups” in IS-IS?
    Mesh groups are used to reduce the flooding of LSPs in highly meshed networks. By designating a subset of routers as “mesh group members,” only these routers will participate in flooding LSPs, reducing the overall flooding overhead.

  10. How does IS-IS handle authentication and security?
    IS-IS supports various authentication mechanisms, including plain-text passwords, MD5 authentication, and more advanced methods like TCP Authentication Option (TCP-AO). Authentication ensures that routing information is exchanged only with trusted routers, preventing unauthorized access and potential security threats.

Mastering IS-IS is essential for network engineers working in service provider environments or large enterprise networks. By understanding the protocol’s key features, hierarchical design, and implementation details, you can ensure efficient routing, scalability, and network performance.

Remember, preparation is key to acing technical interviews. Familiarize yourself with these common IS-IS interview questions and be ready to demonstrate your knowledge and problem-solving skills. Good luck with your interview preparation, and may your journey towards becoming an IS-IS expert be a rewarding one!

Intermediate System to Intermediate System | IS-IS Interview Questions and Answers Part-1


What is Isis protocol in networking?

The IS-IS (Intermediate System – Intermediate System) protocol is one of a family of IP Routing protocols, and is an Interior Gateway Protocol (IGP) for the Internet, used to distribute IP routing information throughout a single Autonomous System (AS) in an IP network.

What are the advantages of OSPF over Isis?

OSPF operates on the top of IP layer whereas ISIS operates over Layer 2. OSPF can support virtual links but ISIS can not support (as it operates on Layer 2 directly). OSPF elects a DR and BDR on broadcast networks which can not be pre-empted however, ISIS elects a single DIS which can be pre-empted.

Can you share your knowledge of routing protocols?

Routing protocols are a set of rules that govern the transmission of data in any network setup. Understanding the fundamentals and technicalities of implementing the right routing protocol is necessary to a network engineer or administrator role.

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