Understanding Selecting Network Protocols

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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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OSPF Network Design Solutions
OSPF Network Design Solutions
ISBN: 1578700469
EAN: 2147483647
Year: 1998
Pages: 200
Authors: Tom Thomas

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