Updated on Apr 2, 2024 by

What Is IS-IS?

IS-IS, initially devised by the International Organization for Standardization (ISO) for its Connectionless Network Protocol (CLNP), serves as a dynamic routing protocol. With the aim of supporting IP routing, the Internet Engineering Task Force (IETF) has expanded and adapted IS-IS in accordance with relevant standards, enabling its utilization in both TCP/IP and Open System Interconnection (OSI) environments. This enhanced iteration of IS-IS, termed Integrated IS-IS or Dual IS-IS, leverages the SPF algorithm for route computation. Renowned for its swift convergence and scalability, IS-IS functions at the data link layer, showcasing robust anti-attack capabilities and facilitating interoperability within expansive network infrastructures.

What is the Necessity of IS-IS?

Prior to the advent of IS-IS, the predominant Interior Gateway Protocol (IGP) was the Routing Information Protocol (RIP). RIP, characterized as a distance-vector routing protocol, is gradually being supplanted by IS-IS due to its sluggish convergence, inclination towards forming routing loops, and limited scalability. IS-IS offers several advantages:

  • Utilization of the TLV format in IS-IS packets facilitates enhanced scalability.

  • Routers can be configured with multiple area addresses, enabling seamless integration of multiple areas.

  • The packet structure remains simple, ensuring efficient interaction among neighbors.

  • IS-IS operates at the data link layer, devoid of dependency on IP addresses.

  • - Employing the SPF algorithm ensures rapid convergence.

  • It caters to expansive networks, such as those of Internet service providers (ISPs).

Basic Concepts of IS-IS

IS-IS Router Classifications

To facilitate the management of extensive routing networks, IS-IS employs a two-tiered structure within a routing domain. This structure allows for the subdivision of large domains into multiple areas. Within an IS-IS network, three levels of routing devices may be present. Typically, Level-1 routers operate within a designated area, Level-2 routers manage communication between areas, and Level-1-2 routers bridge connections between Level-1 and Level-2 devices. The illustration below depicts a typical IS-IS network, where the backbone area encompasses all Level-2 routers within area 1 and Level-1-2 routers across other areas.

IS-IS Network Topology

IS-IS Network Topology

- Level-1 Router:

A Level-1 router oversees intra-area routing exclusively. It establishes neighbor relationships solely with other Level-1 devices and Level-1-2 routers within the same area. Additionally, it maintains a Level-1 link state database (LSDB) containing routing information specific to the local area. For packets bound for other areas, each Level-1 router forwards them to the nearest Level-1-2 router.

- Level-2 Router:

A Level-2 router governs inter-area routing. It has the capability to form neighbor relationships with other Level-2 routers and with Level-1-2 routers in different areas, while managing a Level-2 LSDB comprising inter-area routing data.

All Level-2 routers constitute the backbone network of the routing domain, responsible for inter-area communications. It is imperative for Level-2 routers within the routing domain to be interconnected to ensure the continuity of the backbone network. Only Level-2 routers have the ability to exchange data packets or routing information with routers outside their local area.

- Level-1-2 Router:

A router that operates within both a Level-1 area and a Level-2 area is referred to as a Level-1-2 router. It establishes Level-1 neighbor relationships with Level-1 and Level-1-2 routers within the same area, and Level-2 neighbor relationships with Level-2 and Level-1-2 routers across different areas. Level-1 devices can connect to other areas solely through Level-1-2 devices.

A Level-1-2 router maintains two LSDBs: a Level-1 LSDB utilized for intra-area routing, and a Level-2 LSDB utilized for inter-area routing.

Relationships Between IS-IS Network Types and Network Links

IS-IS operates on both broadcast links, such as Ethernet and token ring, and Point-to-Point (P2P) links, like Point-to-Point Protocol (PPP) connections.

IS-IS networks are categorized into broadcast and P2P networks. Broadcast links can be set to either broadcast or P2P, while P2P links are set only to P2P.

IS-IS Packet Types

IS-IS packets consist of Hello Protocol Data Units (PDUs), Link State PDUs (LSPs), and Sequence Number PDUs (SNPs).

Hello PDU:

Hello PDUs, also known as IS-to-IS Hello PDUs (IIHs), are utilized for establishing and maintaining neighbor relationships. IIH formats vary based on network types. LAN IIHs differ from P2P IIHs in fields like Priority, LAN ID, and Local Circuit ID.

In all IS-IS PDUs, the first eight bytes are public, containing fields like Intradomain Routing Protocol Discriminator and Length Indicator.


LSPs, categorized into Level-1 and Level-2, facilitate the exchange of link-state information. Both types have identical formats, including fields like ATT (Attached bit) and IS Type.


SNPs, comprising Complete Sequence Number Protocol Data Units (CSNPs) and Partial Sequence Number Protocol Data Units (PSNPs), synchronize LSDBs. Broadcast network links and P2P links have distinct SNP usage scenarios. Broadcast networks periodically send CSNPs by the DIS and use PSNPs to request missing LSPs, while P2P links send CSNPs during initial neighbor establishment and use PSNPs for LSDB synchronization.

How Is an IS-IS Neighbor Relationship Established?

Before routing devices can exchange protocol packets for routing purposes, they must establish a neighbor relationship. The process for establishing this relationship varies based on the network type.

Establishing a Neighbor Relationship on a Broadcast Link

On a broadcast link, the neighbor relationship is established through a series of steps:

 Process of establishing a neighbor relationship on a broadcast link

Process of establishing a neighbor relationship on a broadcast link

- DeviceA broadcasts a Level-2 LAN IIH without a neighbor identifier.

- DeviceB receives the packet and sets the neighbor relationship state to Initial.

- DeviceB replies to DeviceA with a Level-2 LAN IIH designating DeviceA as its neighbor.

- DeviceA, upon receiving the reply, sets the neighbor relationship state to Up and responds with its own Level-2 LAN IIH designating DeviceB as its neighbor.

- DeviceB, upon receiving the response, sets the neighbor relationship state to Up, successfully establishing the neighbor relationship.

Establishment of a neighbor relationship on a P2P link

Establishing a neighbor relationship on a P2P link differs from that on a broadcast link, involving either a two-way handshake mechanism or a three-way handshake mechanism.

- Two-way handshake: A routing device unilaterally declares a peer Up upon receiving a Hello packet, establishing a neighbor relationship.

- Three-way handshake: P2P IS-IS Hello PDUs are sent three times to establish a neighbor relationship, similar to the process on a broadcast link.

The two-way handshake mechanism has flaws when multiple links exist between devices. In such cases, the three-way handshake mechanism ensures reliability by confirming packet receipt before establishing the neighbor relationship.

Distinguishing IS-IS from OSPF

The disparities between IS-IS and OSPF encompass several key aspects:

  • Protocol Type: OSPF operates at the IP-layer protocol level, contrasting with IS-IS, which functions at the link layer protocol level.

  • Protocol Extensibility: OSPF extends LSA types for accommodating new requirements, demonstrating moderate extensibility. Conversely, IS-IS exhibits high extensibility owing to its TLV packet structure. For instance, supporting IPv6 merely requires extending the TLV.

  • Application Scope: OSPF is suited for medium-scale networks with up to hundreds of devices, typically found in small- to medium-sized enterprise networks. In contrast, IS-IS caters to large-scale networks, commonly utilized in expansive ISP networks.

  • Routing Algorithm: OSPF employs the SPF algorithm to compute a Shortest Path Tree (SPT) to all destinations based on network topology information disseminated via LSAs. IS-IS also utilizes the SPF algorithm to construct an SPT according to network topology, calculating shortest paths to all destinations. Notably, in IS-IS, the SPF algorithm operates independently in Level-1 and Level-2 databases.

  • Convergence Speed: Both OSPF and IS-IS boast rapid convergence capabilities, typically achieving convergence within 1 second.

  • Area Division: OSPF divides areas based on interfaces, while IS-IS divides areas based on routers.

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