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| 7.1 |  | IS-IS Fundamentals |
| 7.1.4 |  | Integrated IS-IS |

For routing in the ISO CLNS environment, Cisco routers support the IS-IS routing protocol. Routers usually operate as ISs and can exchange reachability information with other ISs using the IS-IS protocol. As an IS, a Cisco router can operate at Level 1 only, at Level 2 only, or at both levels. In the last case, the router can advertise itself at Level 1 as an exit point from the area. Integrated IS-IS allows the IS-IS protocol to propagate routing information for protocols other than CLNP. IS-IS can route CLNP, IP, or both when in dual mode.
IS-IS is the dynamic link-state routing protocol for the OSI protocol stack. As such, IS-IS distributes routing information for routing CLNP data for the ISO CLNS environment. When IS-IS is used strictly for the ISO CLNS environment, it is referred to as ISO IS-IS.

Integrated IS-IS is an implementation of the IS-IS protocol for routing multiple network protocols. Integrated IS-IS tags CLNP routes, upon which IS-IS bases its link-state database, with information regarding IP networks and subnets. IS-IS provides an alternative to OSPF in the IP world, mixing ISO CLNS and IP routing in one protocol. Again, IS-IS can be used purely for IP routing, purely for ISO routing, or for a combination of the two.

Integrated IS-IS is deployed extensively in an IP only environment in the top tier Internet service provider (ISP) networks. The IS-IS working group of the Internet Engineering Task Force (IETF) developed the specification for Integrated IS-IS, RFC 1195. Integrated IS-IS differs from the approach taken by IS-IS routing, where completely independent routing protocols are used for each of the two protocol suites. Integrated IS-IS uses a single integrated protocol for interior routing. The single integrated protocol is used for calculating routes within a routing domain for both protocol suites.

> Note: IS-IS was originally designed for routing DECnet Phase V and was subsequently adopted for ISO CLNP. DECnet is a routed protocol, like IP, IPX, and AppleTalk, used in Digital Equipment Corporation networking. IS-IS was proposed for use with TCP/IP in the late 1980s and early 1990s. IS-IS was ultimately specified for this context in RFC 1195, "Use of OSI IS-IS for Routing in TCP/IP and Dual Environments".

IS-IS is one of the few protocols that provide an integrated framework for concurrent processing of more than one network layer protocol. Other routing protocols, such as OSPF, usually support routing for only one type of Layer 3 protocol. This module will focus mostly on the use of IS-IS in an IP environment.

IS-IS was not designed specifically for routing IP. However, its successful widespread deployment for IP routing on the Internet has led the IETF to revisit RFC 1195. The point is to incorporate proprietary features outside the scope of 1195 designed to improve usability and to provide flexibility and scalability. MPLS traffic engineering stands out as a recent technology that has driven advances in IS-IS feature sets, possibly vendor-specific.

The integrated IS-IS protocol provides dynamic routing for an IP and ISO internetworking environment. Integrated IS-IS has the following features:

- Uses ISO IS-IS to distribute routing information
- Provides ISO and IP routing services
- Routes only within one ISO domain
- Provides link-state distribution of routing information
- Based on the shortest-path-first routing algorithm

Integrated IS-IS provides IP routing with the following capabilities by:

- Defining links with IP addresses, subnets, and metrics
- Forward IP routing information within the PDUs of the ISO IS-IS packets
- Configuring an area to support IP or both IP and CLNP

> Note: For routing in the ISO CLNS environment, Cisco routers also support static CLNS routes as well as the proprietary ISO IGRP routing protocol. ISO IGRP is, as its name suggests, based on Cisco’s Interior Gateway Routing Protocol (IGRP). It uses distance vector technology to propagate routing information. As such, ISO IGRP shares some of the limitations of its IP counterpart, including long convergence times. This is due to periodic updates and long invalid times and hold times.

| Interactive Media Activity Checkbox: IS-IS Terminology Quiz Upon completion of this activity, the student will have a firm grasp on the IS-IS terminology.