The AFI specifies the type of IDI and the network addressing authority responsible
for allocating the values of the IDI.
The IDI specifies both the network addressing domain from which the values of the
DSP are allocated and the network addressing authority responsible for allocating
values of the DSP from that domain.
DSP
The network addressing authority identified by the IDI determines the DSP.
However, what is important is that the DSP includes specific addressing information
for the domain.
IS-IS Addressing Format
The IS-IS protocol divides the NSAP address into three portions; area address,
system ID, and selector (see Figure 22). The area address and system ID, together
with a selector of 0, are referred to as the Network Entity Title (NET). A NET is the
address of the network layer itself and is assigned when you configure an IS into
the OSI network.
Area Address
In the IS-IS protocol, the area address is that portion of the NSAP that includes all
or a portion of the IDP and the portion of the DSP up to the system ID.
The area address is that portion of the NSAP that identifies a specific area within a
domain. This address must be at least 1 octet long and all ESs and ISs in the same
area must have the same area address.
System ID
The system ID is that portion of the NSAP that identifies a specific system within an
area. System IDs must have the following attributes:
v 1 octet to 8 octets in length.
v Equal length throughout the domain. The routers use a default configuration
length of 6 octets.
v Unique for each system throughout the domain.
Selector
The selector is a 1-octet field that acts as a selector for the entity that is to receive
the PDU, for example, the transport layer or the IS network layer itself. The router
sets this field to 0.
┌────────────┬────────────────────────────────────────────────────┐
│ IDP │ DSP │
├────────────┴─────────────┬─────────────────────────┬────────────┤
│ Area Address │ System ID │ Selector │
└──────────────────────────┴─────────────────────────┴────────────┘
Figure 22. IS-IS NSAP Addressing Interpretation
Using OSI/DECnet V
Chapter 9. Using OSI/DECnet V 287
