Administrator Guide

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.
Figure 59. ISO Address Format
Multi-Topology IS-IS
Multi-topology IS-IS (MT IS-IS) allows you to create multiple IS-IS topologies on a single router with separate
databases.
Use this feature to place a virtual physical topology into logical routing domains, which can each support
different routing and security policies.
Intermediate System to Intermediate System 497