how to design the IS-IS network ID

NET is the special form of the Network Service Access Point (NSAP). IS-IS starts only when the NET is configured for the IS-IS process.
NET consists of the following parts:
 Area ID, which is variable (1 to 13 bytes). The area IDs of the router devices in the same area are identical.
 System ID (6 bytes) of this router device. The system ID must be unique in the whole area and backbone area.
 Last byte (SEL), with value 00.
In general, an IS-IS process is configured with only one NET. When you want to redefine an area, to combine it with other areas or divide it into sub-areas for example, you can configure the router with multiple NETs to ensure the correctness of routes.
For this project, all the backbone device belong to one same area, we can set the area ID is 49.
For the system ID, configuring loopback interface addresses based on NETs is recommended to ensure that a NET is unique on the network. If NETs are not unique, route flapping will easily occur.
System ID used in IS-IS can be obtained in the following way: extend each part of the IP address to 3 bits, add 0 to the front of any part that is shorter than 3 bits, divide the extended address into three parts, with each part consisting of four decimal digits, and the reconstructed address is the system ID.
For example, if a loopback 0's IP address is 10.11.101.1, to convert it to a NET, add 0s before each decimal number until the decimal number is three digits long, that is, 010.011.101.001. Then, divide the 12 digits into three segments, each of which consists of four digits, that is, 0100.1110.1001. In this example, the system ID is 0100.1110.1001.

you can find this in product documenation.
IS-IS Address Structure

The network service access point (NSAP) is an address defined by the OSI to locate resources. Figure 4 shows the NSAP address structure. The NSAP is composed of the initial domain part (IDP) and the domain specific part (DSP). The lengths of the IDP and the DSP are variable. The maximum length of the NSAP is 20 bytes and its minimum length is 8 bytes. •The IDP is similar to the network ID in an IP address. It is defined by the ISO and consists of the authority and format identifier (AFI) and the initial domain identifier (IDI). The AFI indicates the address allocation authority and address format, and the IDI identifies a domain.
•The DSP is similar to the subnet ID and host address in an IP address. The DSP consists of the High Order DSP (HODSP), system ID, and NSAP Selector (SEL). The HODSP is used to divide areas, the system ID identifies a host, and the SEL indicates the service type.

Figure 4 IS-IS address structure


•Area Address

The IDP and the HODSP of the DSP identify a routing domain and the areas in a routing domain. Therefore, the combination of the IDP and HODSP is called an area address, which is similar to an area number in OSPF. The area addresses of routers in the same Level-1 area must be the same, while 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 fixed length of the system ID is 48 bits (6 bytes).

In actual applications, a router ID corresponds to a system ID. If a router takes the IP address 168.10.1.1 of Loopback 0 as its router ID, its system ID used in IS-IS can be obtained in the following way:
◾Extend each part of IP address 168.10.1.1 to 3 bits and add 0 to the front of any part that is shorter than 3 bits. Then the IP address is extended as 168.010.001.001.


◾Divide the extended address 168.010.001.001 into three parts, each of which consists of four decimal digits. Then system ID 1680.1000.1001 is obtained.


You can specify a system ID in many ways. You need to ensure that the system ID uniquely identifies a host or a router.


•SEL

The role of an SEL is similar to that of the "protocol identifier" of IP. A transport protocol matche*******L. The SEL is always "00" in IP.


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. Its maximum length is 20 bytes and its minimum length is 8 bytes. When configuring IS-IS on a router, you can configure only a NET instead of an NSAP.

Assume that there is a NET: ab.cdef.1234.5678.9abc.00. In the NET, the area address is ab.cdef, the system ID is 1234.5678.9abc, and the SEL is 00.

The first two digits use the country code 74, and the second four digits use the area code 0123. You can add 0 to the left of the device's loopback address for three digits. For example, if loopback0 192.168.0.1 is added to 0, the last two digits are changed to 1921.6800.0001;. The last two digits are 00 by default. Therefore, the final ID is 74.0123.1921.6800.0001.00. Therefore, the loopback address of each device is different to ensure that the IS-IS IDs are different.

This post was last edited by xiaomumu at 2018-12-27 02:37. The best answer is a good way. Have you solved the problem now?