Typical NSAP Address Structure

The simplest NSAP format used by most companies running IS-IS as their IGP is as follows:

• Area address (must be at least 1 byte)

- AFI set to 49

• Locally administered; thus, you can assign your own addresses.

- Area ID

• The octets of the area address after the AFI.

- Cisco routers require a 6-byte system ID.

- Always set to 0 for a router.

The simplest NSAP format, used by most companies running IS-IS as their interior gateway protocol (IGP), comprises the following:

■ The area address: It must be at least 1 byte, separated into two parts:

— The AFI, set to 49, which signifies that the AFI is locally administered and thus individual addresses can be assigned by the company

— The area identifier (ID), the octets of the area address after the AFI

■ A system ID: Cisco routers compliant with the U.S. Government OSI Profile (GOSIP) version 2.0 standards require a 6-byte system ID.

■ The NSEL: It must always be set to 0 for a router.

Note The NSAP is called the NET when it has an NSEL of 0. Routers use the NET to identify themselves in the IS-IS protocol data units (PDUs).

For example, you might assign 49.0001.0000.0c12.3456.00, which represents the following:

■ System ID of 0000.0c12.3456, the MAC address of a LAN interface

Note The area address is also referred to as the prefix.

Note Some IS-IS documentation uses the terms "area ID" and "area address" as synonyms.

© 2006 Cisco Systems, Inc. The IS-IS Protocol 4-25

Area 1

49.0001.0000.0C22.2222.00

49.0001.0000.0C22.2222.00

49.0001.0000.0C11.1111.00

Area 1

The area address uniquely identifies the routing area, and the system ID identifies each node.

• All routers within an area s must use the same area address.

49.0004.0000.0C88.88

49.0004.0000.0C88.88

• An ES may be adjacent to a router only if they share a common area address.

• Area address is used in

49.0004.0000.0C99.9999.00

Level 2 routing.

The first part of an NSAP is the area address and is associated with the IS-IS routing process. Unlike Open Shortest Path First Protocol (OSPF), an IS-IS router can be a member of only one area.

All routers in an area must use the same area address, which actually defines the area. The area address is used in Level 2 routing.

End systems (ESs) recognize only intermediate systems (ISs) and other ESs on the same subnetwork that share the same area address.

4-26 Building Scalable Cisco Internetworks (BSCI) v3.0

Identifying Systems in IS-IS: System ID

System ID in the address used to identify the IS; it is not just an interface. Cisco supports only a 6-byte system ID.

System ID is used in Level 1 routing and has to be unique within an area.

System ID in the address used to identify the IS; it is not just an interface. Cisco supports only a 6-byte system ID.

System ID is used in Level 1 routing and has to be unique within an area.

System ID has to be unique within Level 2 routers that form the routing domain.

General recommendation: use domain-wide unique system ID.

- This may be MAC (for example, 0000.0c12.3456) or IP address (for example, 1921.6800.0001) taken from an interface.

The 6-byte NSAP system ID must be unique within an area. It is customary to use a MAC address from the router, or, for Integrated IS-IS, to encode an IP address into the system ID. All of the system IDs in a domain must be of equal length. Cisco enforces this OSI directive by fixing the length of the system ID at 6 bytes in all cases.

Level 1 intra-area routing is based on system IDs; therefore, each ES and IS must have a unique system ID within the area.

All Level 2 ISs eventually recognize all other ISs in the Level 2 backbone; therefore, they must also have unique system IDs.

Thus, system IDs should remain unique across the domain. If the system IDs remain unique, there can never be a conflict at Level 1 or Level 2 if, for example, a device moves into a different area.

© 2006 Cisco Systems, Inc. The IS-IS Protocol 4-27

Continue reading here: OSI PDUs

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