Chapter 2 Advanced Routing for FortiOS 5.0 : Intermediate System to Intermediate System Protocol (IS-IS) : IS-IS background and concepts : How IS-IS works : LSP structure
  
LSP structure
It is difficult to fully understand a routing protocol without knowing what information is carried in its packets. Knowing how routers exchange each type of information will help you better understand the IS-IS protocol and will allow you to configure your network more appropriately.
This section provides information on the contents of the IS-IS LSP. LSPs describe the network topology and can include IP routes and checksums.
NSAP and NET
IS-IS routing protocol utilizes ISO network addressing to identify network interfaces. The addresses are known as Network Service Access Points (NSAPs). In general, IS-IS routers consist of only one NSAP, whereas IP addressing requires one IP address per interface.
In IS-IS, the NSAP address is translated into a Network Entity Title (NET), which is the same as the NSAP but can differentiate end systems by way of a byte called the n-selector (NSEL). In order for adjacencies to form in IS-IS, the NSEL must necessarily be set to zero, to indicate “this system”. The total NET can be anywhere between 8 and 20 bytes long due to the support for variable length area addressing.
The following diagram identifies the individual parts of the NSAP, with explanations below.
Table 17: NSAP and NET example
AFI — The Authority and Format Identifier (AFI) specifies the format of the addressing family used. IS-IS is designed to carry routing information for several different protocols. Each entry has an address family identifier that identifies the globally unique Interdomain Part (IDP). For example, 49 is the AFI for private addresses, whereas 47 is the AFI for international organizations.
IDI — The Initial Domain Identifier (IDI) identifies the routing domain within an interconnected network. The length of the IDI is typically determined by the AFI. If you are using an AFI of 49, you do not need to specify an IDI, since the network is private.
HODSP — The High Order Domain-Specific Part (HODSP) identifies the unique address within a specific routing domain. Together, the AFI, IDI, and HODSP define the area address. All of the nodes within an area must have the same area address.
System ID — The System ID represents the 6-8 byte router identifier. The ID could be Media Access Control (MAC) format, as in the example above, or a static length IP address expressed in binary-coded decimal (BCD) format.
NSEL — The n-selector (NSEL), as previously described, identifies the network layer transport service and must always be set to zero for IS-IS NETs.