Content

| 7.2 |  | ISO Addressing |
| 7.2.3 |  | ISO addressing with Cisco routers |

NETs and NSAPs are comprised entirely of hexadecimal digits and must start and end on a byte boundary.

Cisco IOS interprets the NSAP address, from the right-hand end. The last byte is the NSEL and must be specified as a single-byte length preceded by a '.'. A NET definition must set the N-Selector to ’00’.

The preceding six bytes form the system ID. The IOS fixes this length at six bytes. It is customary to code either a Media Access Control (MAC) address from the router or an IP address, such as a loopback address, into the system ID. With Integrated IS-IS, a loopback IP address is commonly used for this purpose. In this case, the system ID is obtained by converting a loopback address: 192.168.111.3 -> 192.168.111.003 -> 1921.6811.1003.

The rest of the address is treated by IOS as the area ID. The area ID can be as small as one byte and as large as 13 bytes. It is customary to use three bytes for the area ID field, with an AFI of one byte such as the 47 in Figure . There are also two additional bytes for area IDs shown as 0001 and the effective area ID is 47.0001. The IOS will attempt to summarize the area ID as far as possible. For example, if an IS-IS network is organized with major areas subdivided into minor areas, and this is reflected in the area ID assignments, then the IOS will do the following:

- Between minor areas, base the route on the whole area ID
- Between major areas, summarize into the area ID portion up to the major area boundary

The following two examples illustrate the use of NSAP addresses with the Cisco IOS:

> Example 1: NSAP 47.0001.aaaa.bbbb.cccc.00

> Here, the IS-IS area ID is 47.0001.
> The system ID is aaaa.bbbb.cccc.
> The NSAP selector byte is 00.

> Example 2: NSAP 39.0f01.0002.0000.0c00.1111.00

> Here, the IS-IS area ID is 39.0f01.0002.
> The system ID is 0000.0c00.1111.
> The NSAP selector byte is 00.