HP 6125XLG R2306-HP 6125XLG Blade Switch Layer 3 - IP Routing Configuration Gu - Page 121
IS-IS area, Level-1 and Level-2
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• Area ID-Has a length of 1 to 13 bytes. • System ID-A system ID uniquely identifies a host or router in the area and has a fixed length of 6 bytes. • SEL-Has a value of 0 and a fixed length of 1 byte. For example, for a NET ab.cdef.1234.5678.9abc.00, the area ID is ab.cdef, the system ID is 1234.5678.9abc, and the 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, make sure the system IDs are the same. IS-IS area IS-IS has a 2-level hierarchy to support large-scale networks. A large-scale routing domain is divided into multiple areas. Typically, a Level-1 router is deployed within an area, a Level-2 router is deployed between areas, and a Level-1-2 router is deployed between Level-1 and Level-2 routers. Level-1 and Level-2 • Level-1 router-A Level-1 router establishes neighbor relationships with Level-1 and Level-1-2 routers in the same area. It maintains an LSDB comprising intra-area routing information. A Level-1 router forwards packets destined for external areas to the nearest Level-1-2 router. Level-1 routers in different areas cannot establish neighbor relationships. • Level-2 router-A Level-2 router establishes neighbor relationships with Level-2 and Level-1-2 routers in the same area or in different areas. It maintains a Level-2 LSDB containing inter-area routing information. All the Level-2 and Level-1-2 routers must be contiguous to form the backbone of the IS-IS routing domain. Level-2 routers can establish neighbor relationships even if they are in different areas. • Level-1-2 router-A router with both Level-1 and Level-2 router functions is a Level-1-2 router. It can establish Level-1 neighbor relationships with Level-1 and Level-1-2 routers in the same area, and establish Level-2 neighbor relationships with Level-2 and Level-1-2 routers in different areas. A Level-1 router can reach other areas only through a Level-1-2 router. The Level-1-2 router maintains two LSDBs, a Level-1 LSDB for intra-area routing and a Level-2 LSDB for inter-area routing. Figure 31 shows one IS-IS network topology. Area 1 is the backbone that comprises a set of Level-2 routers. The other four areas are non-backbone areas connected to the backbone through Level-1-2 routers. 110