Reference 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.
The IS-IS protocol standards are listed in the Standards Compliance chapter.
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:
● area address — within your routing domain or area, each area must have a unique area value. The first byte is called the
authority and format indicator (AFI).
● system address — the router’s MAC address.
● N-selector — this is always 0.
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