Basic concepts

• Intermediate system (IS)—Similar to a router in TCP/IP, it is the basic unit in IS-IS to generate and propagate routing information. In the following text, an IS refers to a router.

• End system (ES)—Refers to a host system in TCP/IP. ISO defines the ES-IS protocol for communication between an ES and an IS. An ES does not participate in the IS-IS processing.

• Routing domain (RD)—A group of ISs exchanges routing information with each other using the same routing protocol in a routing domain.

• Area—A unit in a routing domain. The IS-IS protocol allows a routing domain to be divided into multiple areas.

• Link State Database (LSDB)—All link states in the network forms the LSDB. Each IS has at least one LSDB. The IS uses the SPF algorithm and LSDB to generate its own routes.

• Link State Protocol Data Unit (LSPDU) or Link State Packet (LSP)—Each IS can generate an LSP, which contains all the link state information of the IS.

• Network Protocol Data Unit (NPDU)—A network layer protocol packet in OSI, which is equivalent to an IP packet in TCP/IP.

• Designated IS—On a broadcast network, the designated router is also known as the "designated IS".

• Network service access point (NSAP)—An NSAP is an OSI network layer address. It identifies an abstract network service access point and describes the network address in the OSI reference model.

• NSAP

  As shown in Figure 49, an NSAP address consists of the Initial Domain Part (IDP) and the Domain Specific Part (DSP). The IDP is equal to the network ID of an IP address, and the DSP is equal to the subnet and host ID.

  The IDP includes the Authority and Format Identifier (AFI) and the Initial Domain Identifier (IDI).

  The DSP includes the High Order Part of DSP (HO-DSP), System ID, and SEL, where the HO-DSP identifies the area, the System ID identifies the host, and the SEL identifies the type of service.

  The IDP and DSP are variable in length. The length of an NSAP address varies from 8 bytes to 20 bytes.

• Area address

  The area address comprises the IDP and the HO-DSP of the DSP, which identify the area and the routing domain. Different routing domains cannot have the same area address.

  Typically, a router only needs one area address, and all nodes in the same routing domain must share the same area address. However, a router can have a maximum of three area addresses to support smooth area merging, partitioning, and switching.

• System ID

  A system ID identifies a host or router uniquely. It has a fixed length of 48 bits (6 bytes).

  The system ID of a device can be generated from the Router ID. For example, a router uses the IP address 168.10.1.1 of Loopback 0 as the Router ID. The system ID in IS-IS can be obtained in the following ways:

  • Extend each decimal number of the IP address to 3 digits by adding 0s from the left, like 168.010.001.001;

  • Divide the extended IP address into 3 sections with 4 digits in each section to get the system ID 1680.1000.1001.

  If you use other methods for defining a system ID, always make sure that it can uniquely identify a host or router.

• SEL

  The NSAP Selector (SEL), or the N-SEL, is similar to the protocol identifier in IP. Different transport layer protocols correspond to different SELs. All SELs in IP are 00.

• Routing method

  Because the area information is identified in IS-IS addresses, a Level-1 router can easily identify packets destined to other areas.

  • A Level-1 router makes routing decisions based on the system ID. If the destination is not in the area, the packet is forwarded to the nearest Level-1-2 router.

  • A Level-2 router routes packets across areas according to the area address.

A network entity title (NET) indicates the network layer information of an IS, and does not include transport layer information. It is a special NSAP address with the SEL being 0. The length of the NET is equal to the NSAP, and is in the range of 8 bytes to 20 bytes.

A NET comprises the following parts:

• Area ID—Its length is in the range of 1 to 13 bytes.

• System ID—A system ID uniquely identifies a host or router in the area and has a fixed 6-byte length.

• SEL—It has a value of 0 and a fixed 1-byte length.

For example, a NET is ab.cdef.1234.5678.9abc.00, where, area ID is ab.cdef, system ID is 1234.5678.9abc, and SEL is 00.

Typically, a router only needs one NET, but it can have a maximum of three NETs for smooth area merging and partitioning. When you configure multiple NETs, ensure their system IDs are the same.