Users Guide

Intermediate System to Intermediate System

The intermediate system to intermediate system (IS-IS) protocol that uses a shortest-path-first algorithm. Dell Networking

supports both IPv4 and IPv6 versions of IS-IS.

Topics:

• IS-IS Protocol Overview

• IS-IS Addressing

• Multi-Topology IS-IS

• Graceful Restart

• Implementation Information

• Configuration Information

• IS-IS Metric Styles

• Configure Metric Values

• Sample Configurations

IS-IS Protocol Overview

The IS-IS protocol, developed by the International Organization for Standardization (ISO), is an interior gateway protocol (IGP)

that uses a shortest-path-first algorithm.

NOTE: This protocol supports routers passing both IP and OSI traffic, though the Dell Networking implementation supports

only IP traffic.

IS-IS is organized hierarchically into routing domains and each router or system resides in at least one area. In IS-IS, routers are

designated as Level 1, Level 2 or Level 1-2 systems. Level 1 routers only route traffic within an area, while Level 2 routers route

traffic between areas. At its most basic, Level 1 systems route traffic within the area and any traffic destined for outside the area

is sent to a Level 1-2 system. Level 2 systems manage destination paths for external routers. Only Level 2 routers can exchange

data packets or routing information directly with external routers located outside of the routing domains. Level 1-2 systems

manage both inter-area and intra-area traffic by maintaining two separate link databases; one for Level 1 routes and one for

Level 2 routes. A Level 1-2 router does not advertise Level 2 routes to a Level 1 router.

To establish adjacencies, each IS-IS router sends different protocol data units (PDU). For IP traffic, the IP addressing information

is included in the IS-IS hello PDUs and the link state PDUs (LSPs).

This brief overview is not intended to provide a complete understanding of IS-IS; for that, consult the documents listed in

Multi-Topology IS-IS.

IS-IS Addressing

IS-IS PDUs require ISO-style addressing called network entity title (NET).

For those familiar with name-to-network service mapping point (NSAP) addresses, the composition of the NET is identical to an

NSAP address, except the last byte is always 0. The NET is composed of the IS-IS area address, system ID, and N-selector. The

last byte is the N-selector. All routers within an area have the same area portion. Level 1 routers route based on the system

address portion of the address, while the Level 2 routers route based on the area address.

The NET length is variable, with a maximum of 20 bytes and a minimum of 8 bytes. It is composed of the following:

Intermediate System to Intermediate System 430