Content

| 7.2 |  | ISO Addressing |
| 7.2.4 |  | Identifying systems in IS-IS |

The router assigns a circuit ID of one octet to each interface on the router. In the case of point-to-point interfaces, this is the sole identifier for the circuit, for example 03.

In the case of LAN interfaces, the circuit ID is tagged to the end of the system ID of the designated IS to form a 7-byte LAN ID. For example, 1921.6811.1001.03.

The SNPA is taken from the following:

- MAC address on a LAN interface
- Virtual Circuit ID for X.25 or ATM
- Data link connection identifier (DLCI) for Frame Relay
- High-Level Data Link Control (HDLC) for interfaces

For convenience, the NET restrictions listed in Section 7.2.2 for devices used in IS-IS routing are repeated here as follows:

- All ISs and ESs in a routing domain must have system IDs of the same length. Cisco fixes the system ID length at six bytes.
- All ISs in an area must have the same area address. This defines the area.
- All ESs and Level 1 ISs must have a unique system ID area-wide. Level 1 routing is based on system IDs.
- All Level 2 ISs must have a unique system ID domain wide. It is recommended that, in general, all system IDs remain unique across a domain. That way there can never be a conflict at Level 1 or Level 2 if a device is moved into a different area.
- All ESs in an area will form an adjacency with a Level 1 IS on a shared media segment if they share the same area address. ESs recognize only ESs and ISs on the same subnetwork that share the same area address.
- If multiple NETs are configured on the same IS, they must all have the same system ID.

The following are several techniques used for creating unique system IDs:

- Start numbering 1, 2, 3, 4, and so on.
- Use MAC addresses.
- Convert and use the loopback IP address, 192.168.11.1 -> 192.168.011.001 -> 1921.6801.1001. This is how most ISPs define system IDs.

The practice of using a modified loopback IP address as the system ID is becoming outdated due to the dynamic hostname feature available in Cisco IOS releases beginning with 12.1.

IS-IS and ES-IS PDUs contain variable-length fields, depending on the function of the PDU. Each field contains a type code and length, followed by the appropriate values. For that reason, the abbreviation TLV is used for Type, Length, and Value fields. The dynamic hostname feature is specified in RFC 2763. It uses a new TLV, namely TLV 137, to map the hostname of the router to the system ID. In the LSP flooding process, a dynamic distribution of hostname to NET mappings takes place. This prevents having to maintain a huge database of static mappings for system IDs on all the IS-IS routers. Maintaining such a database has been a very real issue faced by ISPs running Integrated IS-IS.

Finally, when routing CLNS, request an official NSAP address for use in addressing areas. When only routing IP, just use AFI 49, then the area numbering appears as 49.0001, 49.0002, 49.0003, and so on.

The 1-byte N-selectors are set to 00, indicating these are NETs. The 6-byte system IDs are unique across the network. Additionally, the 3-byte area IDs are common to each area and distinct between areas. -