NSAP Format

NSAP addresses are not fixed in length and can be up to 160 bits (20 bytes) long compared to the fixed 32 bits (4 bytes) of IP addresses. ISO 8348/AD2 specifies a hierarchical scheme for defining global and public NSAP addresses. The following are the seven top-level addressing domains:

• X.121 ” International plan for public data networks

• ISO DCC ” Data Country Code

• F.69 ” Telex

• E.163 ” Public Switched Telephone Network

• E.164 ” ISDN

• ISO 6523 ” International Code Designator (ICD) for organizations

• Local ” For local use only within network domain

Even though format details differ for each top-level domain, all NSAP addresses conform to the generic format shown in Figure 4-3.

Figure 4-3. NSAP address format.

The NSAP format shows two main components : the initial domain part (IDP) and the domain-specific part (DSP). The IDP is further broken down into the authority and format identifier (AFI) and the initial domain identifier (IDI). The DSP consists of the high order DSP, ID, and SEL fields. The ID field is for the SysID of the node and the SEL represents the NSAP selector (NSEL). Both the SysID and NSEL were discussed in the preceding section.

The details of the naming and functions of all various fields in the NSAP are not discussed here because they are irrelevant to the subject matter of this book. However, the AFI and IDI fields are of great interest, so they are discussed in detail here. The AFI indicates the top-level addressing domain associated with the NSAP and the syntax of the DSP section. Possible values of the AFI range from 0 to 99 decimal values. The top-level address domains sponsor various subdomains, which are assigned a value in the IDI field. Each top-level domain specifies its own format for the IDI field. For example, the ISO 6523 ICD address domain has a four-digit format, whereas the ISO DCC uses a three-digit format for the data country code. Examples of IDI values for ISO 6523 ICD subdomains used by the United States government follow:

• U.S. government civilian organizations ” 0005

• U.S. Department of Defense ” 0006

The IDI value of 0005 refers to U.S. government civilian organizations that conform to the U.S. government standard known as Government Open System Interconnection Profile (GOSIP).

As said before, the value of the AFI also determines the syntax of the DSP. The DSP syntax can be in binary octets, decimal digits, or even characters .

Table 4-1 shows the AFI values for various top-level address domains and DSP syntax types. For example, AFI 47 refers to the ISO 6523 ICD addressing domain and indicates a binary syntax for the DSP. Similarly, AFI 39 refers to the ISO DCC addressing domain and binary syntax for the DSP.

Table 4-1. AFI Values for Address Domains and DSP Syntax Types

Address Domain	DSP Syntax
	Decimal	Binary	Character
X.121	36	37
ISO DCC	38	39
F.69	40	41
E.163	42	43
E.164	44	45
ISO 6523 ICD	46	47
Local	48	49	50

Example 4-1 shows an ISO 6523 address that conforms to the GOSIP standard.

Example 4-1 A Complete IS0 6523 NSAP

 47.0005.80123456000089AB001.AABBCCDDEEFF.00  ^    ^                       ^                               ^              ^ AFI   IDI                   AREA                             SYSID          NSEL 

Example 4-1 shows a complete 20-byte NSAP with key fields in the layout delineated by grouping related characters. The following five major fields can be easily discerned:

• AFI (47) ” Address domain indicating binary DSP syntax

• IDI (0005) ” GOSIP IDI

• Area (80123456000089AB0001) ” Area information with hierarchy details

• SysID ” 6-byte System Identifier

• NSEL ” NSAP selector specifying routing layer as network service user

Simplified NSAP Format

As shown in Figure 4-4, the various fields in the NSAP format can be grouped into three main sections: Area ID, System ID, and NSEL. This interpretation of the NSAP format reduces its seeming complexity. The resulting structure is referred to as the simplified NSAP format. Future discussions of the NSAP in this chapter are based on the simplified NSAP format.

Figure 4-4. Simplified NSAP format.

The Area ID field consists of the AFI (first byte) and all subsequent fields up to the beginning of the System ID section. The Area ID field has variable length. The length of the System ID field is specified to be 1 to 8 bytes. The NSEL is the last byte. The maximum size of the NSAP address in the simplified format remains 20 bytes.

NOTE

Most current implementations of IS-IS, including the Cisco implementation, have adopted a fixed-size , 6-byte System ID length in compliance with the U.S. GOSIP version 2.0 standard.

Considering the GOSIP-specified 6-byte length of the System ID field and the 1-byte NSEL field, the Area ID may therefore vary between 1 and 13 bytes. Because only 1 byte is sufficient to define the Area ID, the smallest length of an NSAP on a Cisco router is 8 bytes.

For IP applications, it is sufficient to define NSAPs as simple as possible by allocating 1 byte for the AFI, at least 2 bytes for the actual area information, 6 bytes for the System ID, and 1 byte for the NSEL for a total minimum of 10 bytes. Example 4-2 shows a 10-byte NSAP, based on this recommendation. NSAPs are configured in hexadecimal format from the IOS command-line interface (CLI) with the leading AFI byte (decimal value) and trailing NSEL byte (00) delineated by dots (periods). The remainder of the address between the AFI and NSEL is broken down into 4-digit (2 bytes in Hex) groups and separated by dots. In comparison, IP addressing uses a different format, called dotted -decimal representation. See Chapter 1 for a review on IP addressing.

Example 4-2 Simple NSAP Address

 49.0001.0000.0000.0001.00     ^                    ^              ^ Area ID                SysID           NSEL 

When using IS-IS for IP routing, you can follow the simplified NSAP format to create a simple Area ID without regard for other details, such as IDI, domain, and details of the area information. In the preceding example, an AFI value of 49 is prefixed to the intended area information (0001) to form the Area ID (49.0001). Recall from Table 4-1 that the AFI value of 49 is designated for local private use similar to the reserved private address space specified in RFC 1618. The next characters after the AFI are in hexadecimal format; the first 12 digits represent the 6-byte SysID and the last 2 digits represent the NSEL byte.

Although 10-byte-long NSAPs are sufficient for IP routing purposes, most service providers using IS-IS still configure 20-byte-long addresses on their routers whether they use conjectured addresses (with AFI 49) or addresses from the public space (obtained from one of the top-level addressing domains). For routing on the Internet, IS-IS (and certainly any other IGP such as OSPF) is confined to the local domain (or autonomous system). The Border Gateway Protocol ( BGP ) is used instead for sharing routing information between autonomous systems. In practice, the NSAPs configured on IP routers for IS-IS routing do not need to be globally unique or even 20 bytes long. This is explained further in the next section.