1-3The system ID of a device can be generated from the Router ID. For example, a router uses the IPaddress 168.10.1.1 of Loopback 0 as the Router ID, and the system ID in IS-IS can be obtained in thefollowing way:z Extend each decimal number of the IP address to 3 digits by adding 0s from the left, like168.010.001.001;z Divide the extended IP address into 3 sections with 4 digits in each section to get the system ID1680.1000.1001.There are other methods to define a system ID. The principle is to make sure it can uniquely identify ahost or router.4) SELThe NSAP Selector (SEL), or the N-SEL, is similar to the protocol identifier in IP. Different transportlayer protocols correspond to different SELs. All SELs in IP are 00.5) Routing methodBecause the area information is identified in IS-IS addresses, a Level-1 router can easily identifypackets destined to other areas.z A Level-1 router makes routing decisions based on the system ID. If the destination is not in thearea, the packet is forwarded to the nearest Level-1-2 router.z A Level-2 router routes packets across areas according to the area address.NETA network entity title (NET) indicates the network layer information of an IS and does not includetransport layer information. It is a special NSAP address with the SEL being 0. Therefore, the length ofthe NET is equal to the NSAP and is in the range 8 bytes to 20 bytes.Generally, a router only needs one NET, but it can have three NETs at most for smooth area mergingand partitioning. When you configure multiple NETs, make sure their system IDs are the same.For example, a NET is ab.cdef.1234.5678.9abc.00, where,Area = ab.cdef, System ID = 1234.5678.9abc, and SEL = 00.IS-IS AreaTwo-level hierarchyIS-IS has a two-level hierarchy to support large scale networks. A large scale routing domain is dividedinto multiple Areas. Typically, a Level-1 router is deployed within an area, a Level-2 router is deployedbetween areas, and a Level-1-2 router is deployed between Level-1 and Level-2 routers.Level-1 and Level-21) Level-1 routerA Level-1 router establishes neighbor relationships with Level-1 and Level-1-2 routers in the same area.The LSDB maintained by the Level-1 router contains the local area routing information. It directs thepackets destined for an outside area to the nearest Level-1-2 router.2) Level-2 routerA Level-2 router establishes neighbor relationships with the Level-2 and Level-1-2 routers in the sameor in different areas. It maintains a Level-2 LSDB which contains inter-area routing information. All theLevel-2 and Level-1-2 routers must be contiguous to form the backbone of a routing domain.
