Reference Guide

21
Intermediate System to Intermediate System
Intermediate system to intermediate system (Is-IS) is supported on the Z9000 platform.
• IS-IS is supported on the Z9000 with FTOS version 9.0.0.0.
• The IS-IS protocol is an interior gateway protocol (IGP) 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.
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.
The following illustration is an example of the ISO-style address to show the address format IS-IS uses. In this example,
the first five bytes (47.0005.0001) are the area address. The system portion is 000c.000a.4321 and the last byte is always 0.
345