Only after configuring the basic functions can an IS-IS network be established.

Prerequisites

Before configuring the basic functions of IS-IS, the following task must be completed:

Configure the IP addresses of the interfaces to ensure network layer reachability between adjacent nodes.

Configure Network Entity Title (NET)

Network Entity Title (NET) is a key component of the addressing scheme. It uniquely identifies each Intermediate System (IS) within the IS-IS routing domain.

The NET consists of three parts:

• Area ID: The length of the Area ID can vary from 1 to 13 bytes.

• System ID: This part has a fixed length of 6 bytes.

• NSAP Selector (SEL): The last byte, different transport protocols correspond to different SELs. For IP, the SEL is always 00.

The set protocols isis area-tag <text> [vrf <vrf-name>] network-entity <network-entity> command configures network entity title (NET) provided in ISO format. When configuring the NET parameter in IS-IS, ensure that these three components are properly defined according to your network requirements.

Define the IS-IS instance by specifying the area-tag <text>. This area-tag <text> helps identify the IS-IS instance on the router.

The following command defines the NET as 32.0001.0040.0220.2030.00. The system ID is 0040.0220.2030 and the region ID is 32.0001.

Enable IS-IS Instance on the Interface

After configuring the IS-IS instance, to ensure the proper operation of the IS-IS protocol, you also need to enable IS-IS on the link interfaces where IS-IS is running and associate them with the specified instance. The set protocols isis area-tag <text> interface <interface-name> <ipv4-routing | ipv6-routing> command enables IS-IS instance on a specific L3 interface.

The following command enables IS-IS instance on a specific interface.

Configure the Level of the IS-IS Switch

IS-IS switches can be categorized into the following three levels:

• Level-1 switches: Responsible for routing within an area, they only form neighbor relationships with Level-1 and Level-1-2 switches belonging to the same area. Level-1 switches must connect to other areas through Level-1-2 switches.

• Level-2 switches: Responsible for routing between areas, they can form neighbor relationships with Level-2 switches or Level-1-2 switches from other areas. All Level-2 switches constitute the backbone network of the routing domain, facilitating communication between different areas. Level-2 switches within the routing domain must be contiguous to ensure the continuity of the backbone network.

• Level-1-2 switches: They can form Level-1 neighbor relationships with Level-1 and Level-1-2 switches within the same area, as well as Level-2 neighbor relationships with Level-2 and Level-1-2 switches from other areas.

The set protocols isis area-tag <text> [vrf <vrf-name>] is-type <level-1 | level-1-2 | level-2-only> command configures the level of the IS-IS switch.

It is advisable for users to complete the configuration of the switch level when configuring IS-IS before network operation.

If the switch level is not specified using this command during the IS-IS configuration process, the router defaults to being a Level-1-2 router. This means it participates in both Level-1 and Level-2 routing calculations, maintaining two LSDBs for Level-1 and Level-2.

The following command configures the level of the IS-IS switch.