Content

| Overview

In recent years, the Intermediate System-to-Intermediate System (IS-IS) routing protocol has become increasingly popular, with widespread usage among Service Providers. IS-IS enables very fast convergence and is very scalable. It is also a very flexible protocol and has been extended to incorporate leading edge features such as Multiprotocol Label Switching Traffic Engineering (MPLS/TE).

The features of IS-IS include the following:

- Hierarchical routing
- Classless behavior
- Rapid flooding of new information
- Fast Convergence
- Very scalable
- Flexible timer tuning

The Cisco IOS implementation of IS-IS also supports the following features:

- Multi-area routing
- Route-leaking
- Overload-bit

IS-IS is an Open System Interconnection (OSI) routing protocol originally specified by International Organization for Standardization (ISO) 10589. IS-IS is a dynamic, link-state, intradomain, interior gateway protocol (IGP). The protocol is designed to operate in an OSI Connectionless Network Service (CLNS) environment. IS-IS selects routes based upon a cost metric assigned to links in the IS-IS network. The cost is an arbitrary value assigned by a network engineer as the value of the path to a neighbor router.

A two-level hierarchy is used to support large routing domains. A large domain can be administratively divided into areas. Each system resides in exactly one area. Routing within an area is referred to as Level 1 routing. Routing between areas is referred to as Level 2 routing. A Level 2 Intermediate System (IS) keeps track of the paths to destination areas. A Level 1 IS keeps track of the routing within its own area. For a packet destined for another area, a Level 1 IS sends the packet to the nearest Level 2 IS in its own area, regardless of the level of the destination area. Then the packet travels by way of Level 2 routing to the destination area, where it may travel by way of Level 1 routing to the destination. It should be noted that selecting an exit from an area based on Level 1 routing to the closest Level 2 IS might result in suboptimal routing.

On broadcast multi-access media, a Designated Intermediate System (DIS) is elected and conducts the flooding over the media. The DIS is analogous to the designated router in OSPF, even though the details including the election process and adjacencies within a multi-access media differ significantly. The DIS is elected by priority. The highest priority becomes the DIS. This is configurable on an interface basis using the isis priority command. In the case of a tie, the router with the highest subnetwork point-of-attachment address (SNPA) will become the DIS. In the case of Ethernet, the SNPA address is just the MAC address.

All of these concepts are discussed in this module, beginning with an introduction to OSI protocols.