IS-IS (Intermediate System-to-Intermediate System)

I have included this section to raise more appreciation for the IS-IS routing protocol. In Open System Interconnection (OSI) CLNS environments, CLNP provides a network layer service to peer CLNS entities. CLNP can be seen as the ISO equivalent of (connectionless) IP datagram delivery.

The following dynamic routing approaches can be used to route CLNP:

  • IS-IS (Intermediate System-to-Intermediate System)

  • ES-IS (End System-to-Intermediate System)

  • Cisco proprietary ISO IGRP (Interior Gateway Routing Protocol), capable of routing between CLNP domains

Static CLNS routes can be configured as well. All of these protocols travel via native Layer 2; they do not travel on top of IP, in contrast to OSPF.

An intermediate system represents a router in OSI lingo, and an end-system represents a workstation. IS-IS is a dynamic classless OSI link-state routing protocol based on the Dijkstra SPF algorithm. It essentially originated from DECnet Phase V routing. It supports a two-level area hierarchy, similar to OSPF, resembling a backbone (Level 2) and leaf areas (Level 1) to support large routing domains. IS-IS does not have a backbone area like the OSPF area 0. A contiguous collection of Level 2 routers resembles the backbone. In contrast to OSPF, the border between areas is on the link that connects two routers that are located in different areas.

IS-IS is popular among Tier-1/2 carriers and some ISPs. Originally, it was designed for CLNS, but it was later extended to support IP as a network layer protocol as well. IS-IS with IP support is referred to as integrated or dual IS-IS.

Disadvantages of IS-IS

The point that makes IS-IS difficult to grasp is the issue of the network service access point (NSAP) addresses required for node identification in combination with CLNP as an additional network layer protocol. In contrast to IP, one intermediate system in general has only one NSAP address. For more detailed information about addressing and IS-IS operation, see the "Recommended Reading" section at the end of this chapter.

Advantages of IS-IS

One of the big advantages of IS-IS is its exceptional convergence behavior in combination with its scalability to support large areas of several hundred intermediate systems without considerable SPF performance degradation. One can argue whether IS-IS TLVs (Type-Length-Values) or OSPF LSAs (link-state advertisements) are more complicated to understand. IS-IS appears simpler in that respect because the area concept is more straightforward. IS-IS does not implement virtual links. Cisco IOS Software provides some additional features such as route leaking, overload bit, and multi-area routing.

IS-IS is an elegant protocol and in some aspects easier to grasp and easier to manage than OSPF. I consider the only reason for its lack of popularity among noncarrier staff the "strange" NSAP addresses it depends on, its relationship with CLNS/CLNP, and the fact that it uses Layer 2 for transport. Cisco offers an excellent implementation of IS-IS. IS-IS can be deployed or additionally used for "IP out-of-band" management of network nodes, because of the integrated/dual character of IS-IS and its independence of a Layer 3 network protocol for transport.

Relevant IS-IS Standards

To raise more appreciation for IS-IS design and benefits, I have included an exhaustive collection of relevant standards:

  • ISO 7498, "Open System Interconnection Model"

  • ISO 10589, "ISO IS-IS"

  • RFC 1142, "OSI IS-IS Intra-Domain Routing Protocol"

  • RFC 2763, "Dynamic Hostname Exchange Mechanisms for IS-IS"

  • RFC 2966, "Domain-Wide Prefix Distribution with Two-Level IS-IS"

  • RFC 2973, "IS-IS Mesh Groups"

  • RFC 1195, "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments (Integrated IS-IS)"

  • ISO 9542/RFC 995, "ISO ES-IS"

  • ISO 8473/RFC 994/RFC 1069, "ISO CLNS/CLNP"

  • ISO 8348-Ad2/RFC 1629, "NSAP Address Formats"

  • RFC 3559, "Reserved Type, Length and Value (TLV) Codepoints in Intermediate System to Intermediate System"

  • draft-ietf-isis-traffic-04.txt, "TE Extensions to IS-IS" (new TLVs and sub-TLVs)

Current IS-IS Developments

After a quiet period, IS-IS development is quite active again. The following list introduces aspects of current IS-IS evolution:

  • Management Information Bases (MIBs)

  • IPv6

  • Cryptographic authentication

  • TE extensions

  • Mesh groups