What Is the Specific Use of the NSEL Field in the NSAP Address of ISIS

Hi,

 IS-IS address structure

• Area address

  An IDP and HODSP of the DSP can identify a routing domain and the areas in a routing domain; therefore, the combination of the IDP and HODSP is referred to as an area address, equal to an area ID in OSPF. An area address is used to uniquely identify an area in a routing domain. The area addresses of routers in the same Level-1 area must be the same, whereas the area addresses of routers in the Level-2 area can be different.

  In general, a router can be configured with only one area address. The area address of all nodes in an area must be the same. In the implementation of a device, an IS-IS process can be configured with a maximum of three area addresses to support seamless combination, division, and transformation of areas.

• System ID

  A system ID uniquely identifies a host or a router in an area. In the device, the length of the system ID is 48 bits (6 bytes).

  A router ID corresponds to a system ID. If a device uses the IP address of Loopback 0 (192.168.1.1) as its router ID, its system ID used in IS-IS can be obtained by performing the following steps:

  There are many ways to specify a system ID. Whichever you choose, ensure that the system ID uniquely identifies a host or a device.

  

  If the same system ID is configured for more than one device on the same network, network flapping may occur. To address this problem, IS-IS provides the automatic recovery function. With the function, if the system detects an IS-IS system ID conflict, it automatically changes the local system ID to resolve the conflict. The first two bytes of the system ID automatically changed by the system are Fs, and the last four bytes are randomly generated. For example, FFFF:1234:5678 is such a system ID. If the conflict persists after the system automatically changes three system IDs, the system no longer resolves this conflict.

  • Extend each part of the IP address 192.168.1.1 to 3 digits and add 0 or 0s to the front of the part that is shorter than 3 digits.

  • Divide the extended address 192.168.001.001 into three parts, with each part consisting of 4 decimal digits.

  • The reconstructed 1921.6800.1001 is the system ID.

• SEL

  The role of an SEL (also referred to as NSAP Selector or N-SEL) is similar to that of the "protocol identifier" of IP. A transport protocol matches an SEL. The SEL is "00" in IP.

• NET

  A Network Entity Title (NET) indicates the network layer information of an IS itself and consists of an area ID and a system ID. It does not contain the transport layer information (SEL = 0). A NET can be regarded as a special NSAP. The length of the NET field is the same as that of an NSAP, varying from 8 bytes to 20 bytes. For example, in NET ab.cdef.1234.5678.9abc.00, the area is ab.cdef, the system ID is 1234.5678.9abc, and the SEL is 00.

  In general, an IS-IS process is configured with only one NET. When areas need to be redefined, for example, areas need to be combined or an area needs to be divided into sub-areas, you can configure multiple NETs.

  

  A maximum of three area addresses can be configured in an IS-IS process, and therefore, you can configure only a maximum of three NETs. When you configure multiple NETs, ensure that their system IDs are the same.

  The routers in an area must have the same area address.