IS-IS Protocol Basics [Features and Characteristics]

IS-IS protocol (Intermediate System-to-Intermediate System protocol) is a link-state Interior Gateway Protocol protocol. (Don’t confuse−ISIS, IS IS, or IS-IS, all are same in term of routing protocol. Since it is a link-state routing protocol, it offers several advantages over distance-vector protocols. It was originally designed for use as a dynamic routing protocol for ISO Connectionless Network Service (CLNP). In the beginning, the IS-IS protocol was very popular and most of the Service Providers were used it. At that time, it was well compatible with the IPX/SPX protocol suite designed by Novell NetWare. After few years of its development, it has been redesigned to carry IP prefixes along with the CLNP.

The convergence time of IS-IS protocol is very fast and it can easily be scaled in large networks. It is also a very flexible protocol and has been prolonged to incorporate important superiority features such as MPLS (Multi-Protocol Label Switching). The basic features of the IS-IS protocol remain same as OSPF, however; it provides some additional features that are missing by the OSPF protocol.

Features of IS-IS Protocol

The list of features provided by the IS-IS protocol is pretty long. The following are some of the key features provided by the IS-IS protocol.

  • It supports hierarchical routing structure.
  • It is a classless routing protocol, hence supports CIDR, VLSM, and discontinuous network.
  • It floods new information in the network so rapidly.
  • Its convergence time is very fast.
  • It is very scalable routing protocol.
  • It provides flexible timer tuning.
  • It uses Cost as the default metric but can also use optional metrics, such as Delay, Expense, and Error.
  • It uses Dijkstra Shortest Path First algorithm to calculate the best path.
  • It is less widely implemented on router platforms.
  • It runs on data link layer of OSI model.
  • It uses Network Entity Title (NET ─the address of the network entity itself) to identify each router on a network.
  • It uses IS-IS Hello (IIHs) PDUs to form adjacencies between routers.
  • It supports authentication that allows configuring a password for a specified link, for an area/entire domain.

IS-IS Protocol Levels

As we mentioned earlier, “IS-IS provides the hierarchal structure”. To make a hierarchal structure, IS-IS protocol uses layered hierarchy. IS-IS protocol has two layered hierarchy: Level 1 and Level 2.

IS-IS Router Types

A router in an IS-IS-based network can be categorized into three types: Level 1 (L1), Level 2 (L2), and Level 1/Level 2 (L1/L2).

Level 1 IS-IS Router

An L1 router has neighbors only in the same area. It has a level 1 Link-State Database (LSDB) containing all routing information for the area. The L1 router uses the L1/L2 router as a default gateway to reach destinations, similarly as an OSPF stub router uses the ABR.

Level 2 IS-IS Router

An L2 router may have neighbors in the same or even in the other areas. It has a level 2 LSDB containing all routing information about inter-area. If the L1 and L2 routers are in the same link, they cannot form an adjacency.

Level 1/ Level 2 IS-IS Router

An IS-IS router can act as an L1 and an L2 router at the same time. These routers are referred as L1/L2 routers. An L1/L2 router may have neighbors of any area. In addition, an L1/L2 router has two separate LSDBs: level-1 LSDB for L1 routers and level-2 LSDB for L2 routers. All Cisco routers are by default act as L1/L2 routers. The following figure displays the basic topology of ISIS routers.

IS-IS Protocol Router Levels

In this post, we have discussed the basics of IS-IS protocol. The entire concept of the IS-IS protocol is so lengthy. Hence, we decided to cover it into multiple posts. In the upcoming posts, we will cover more about the IS-IS protocol.

What’s Next?

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