IS-IS Terminology¶

IS-IS has some common terms used throughout this section which can be confusing for those unfamiliar with the protocol:

Designated Intermediate System (DIS):: The designated router for a given broadcast network.
Network Service Access Point (NSAP) Address:: A network address, similar to a layer 3 IP address, which uniquely identifies a node.
Protocol Data Unit (PDU):: A single unit of information sent between peers to exchange data. PDUs contain headers and data required for the protocol to operate.
Type Length Value (TLV):: A construct which allows IS-IS to transmit many different types of information within its PDUs. These constructs make IS-IS modular, and allow it to be quickly updated to support new features and protocols.

IS-IS PDU Types¶

Hello:: PDUs exchanged initially and periodically so routers can establish an IS-IS adjacency with neighbors.
Link State PDU (LSP):: Messages which contain routing data, expressed using TLVs, that IS-IS exchanges between neighbors.
Complete Sequence Number PDU (CSNP):: PDU transmitted periodically by the DIS which contains complete LSP metadata.
Partial Sequence Number PDU (PSNP):: PDU requesting the DIS to transmit specific LSP messages to help resolve mismatches between CSNP data and a the local routing database.

Levels¶

IS-IS uses levels to make decisions when locating the shortest path between two areas and networks within areas.

Level 1:: Level 1 routers exist in the outlying portions of the network, within a single area, and are somewhat similar to “stub” area routers in OSPF. Level 1 routers route traffic to destinations within their own area or using their closest level 2 router for destinations in other areas.
Level 2:: Level 2 routers connect to other Level 2 routers and determine how to reach destinations when routing between areas. Level 2 routers are similar to “backbone” (area 0) routers in OSPF.
Level 1-2:: Level 1-2 routers live in both areas and act as a border between an area and the backbone.

Network Entity Title (NET)¶

The Network Entity Title (NET) acts as a router ID for an IS-IS router. It is potentially one of the most confusing attributes as relatively few users are familiar with the NSAP address format.

The following table (Network Entity Title (NET) Example) shows a breakdown of an example IS-IS NET value, 49.0001.0102.2203.0004.00:

Network Entity Title (NET) Example¶
AFI		Area ID		System ID		NSEL
49	.	0001	.	0102.2203.0004	.	00

As shown by the example, an IS-IS NET is composed of four parts:

Authority and Format Identifier (AFI):

For the purposes of IS-IS, this should always remain at a value of 49 which indicates the address is private.

Area ID:

Specifies the area to which this router belongs, formatted as four digits. In this example, the router is a part of area 1, formatted as 0001.

In the majority of cases this value will remain the same on each router, except in cases where a network contains multiple IS-IS areas.

System ID:

The portion of the address which uniquely identifies a router. This value must be unique between all routers on the entire network.

Typically this is formed from an IP address unique to the router, such as its loopback address, management address, or other internal network address.

The easiest way to form the system ID is to take that IP address, zero pad the start of each octet so it is three digits, and then reformat the address with a period every four digits instead of every three.

This table shows how to use those steps to translate an IP address of 10.222.30.4 to a system ID of 0102.2203.0004:

System ID Creation Process¶
Step	Result
Starting IP address	`10.222.30.4`
Leading zero padding	`010.222.030.004`
Remove periods	`010222030004`
Split into three groups	`0102.2203.0004`

Network Selector (NSEL):

For IS-IS this value will always be 00 which tells IS-IS that this NET is used for routing purposes.