Understanding Selecting Network Protocols | OSPF Network Design Solutions

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

IS-IS is an OSI link-state hierarchical routing protocol based on work originally done at Digital Equipment Corporation (Digital) for DECnet/OSI (DECnet Phase V). This protocol floods the network with link state information to build a complete, consistent picture of network topology.

The International Organization for Standardization (ISO) has developed the following routing protocols for use in the Open Systems Interconnection (OSI) protocol suite:

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

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

• Interdomain Routing Protocol (IDRP)

Because none of the OSI-based protocols have been covered, it is important to mention them so that you know they exist. For more information on IDRP or ES-IS, I would recommend starting with the RFCs, which can be found at http://www.internic.net.

The American National Standards Institute (ANSI) X3S3.3 (network and transport layers) committee was the motivating force behind ISO standardization of IS-IS, which was originally developed to route in ISO Connectionless Network Protocol (CLNP) networks. A version has since been created that supports both CLNP and Internet Protocol (IP) networks. It is usually referred to as Integrated IS-IS; this is the version we will be discussing.

OSI routing protocols are summarized in several ISO documents; those dealing with IS-IS are as follows:

ISO 10589: Standards definition for IS-IS

RFC 1195: Intermediate IS-IS

Both are available via the Internet at the appropriate standards home page or at Cisco Systems’ home page:

    ftp://ftp-eng.cisco.com/RFC/ISO/iso10589.ps    ftp://ftp-eng.cisco.com/RFC/rfc/rfc1195.ps

IS-IS Specific Terminology

Integrated IS-IS uses some interesting terminology; the following briefly defines some useful terms:

• End System (ES). An ES refers to any non-routing network component, such as a workstation.

• Intermediate System (IS). Refers to a router.

• Area. A specified group of contiguous networks and hosts.

• Domain. A group of connected areas.

• Level 1 routing. Refers to routing within an area (level 1).

• Level 2 routing. Refers to routing between level 1 areas.

To simplify router design and operation, IS-IS distinguishes between Level 1 and Level 2 Intermediate Systems (IS):

• Level 1 ISs. Level 1 Intermediate Systems (IS) communicate with other Level 1 ISs in the same area.

• Level 2 ISs. Level 2 ISs route between Level 1 areas and form an intradomain routing backbone.

Hierarchical routing simplifies backbone design because Level 1 ISs only need to know how to get to the nearest Level 2 IS. The backbone routing protocol can also change without affecting the level 2 IS routing protocol. Figure 3-2 demonstrates the relationship between areas and the different levels of routing.

Figure 3-2 IS-IS area and domain routing.

Integrated IS-IS Overview

Integrated IS-IS is a version of the OSI IS-IS routing protocol that uses a single routing algorithm to support more network layer protocols than just CLNP. Integrated IS-IS is sometimes called Dual IS-IS, named after a version designed for IP and CLNP networks.

Several fields are added to IS-IS packets to enable IS-IS to support additional network layers. These fields inform routers about the following:

• The accessibility of network addresses from other protocol suites

• Which protocols are supported by which routers.

• Other information required by a specific protocol suite

Integrated IS-IS implementations send only one set of routing updates, making it more efficient than two separate implementations.

IS-IS Protocols

As in other powerful protocols, such as OSPF, the IS-IS protocol contains subprotocols. The two most important are the Hello protocol and the Flooding protocol. The Hello protocol is used to discover and to elect a Designated Router on broadcast links. The Flooding protocol is used to propagate the link state records within the areas. Each subprotocol has a very important function within the IS-IS protocol. These functions are detailed as follows:

• Hello protocol. This protocol carries the identification information, which characterizes a router as a Level 1, Level 2, or both.

• Flooding protocol. This protocol is used to propagate the link state records within the areas to ensure that proper routing updates take place.

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