Is-Is Area - HP MSR Series Configuration Manual

System ID—A system ID uniquely identifies a host or router in the area and has a fixed length of
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6-byte.
SEL—Has a value of 0 and a fixed length of 1-byte.
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For example, for a NET is 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 their 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
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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-2 router—A Level-2 router establishes neighbor relationships with the Level-2 and Level- 1 -2
routers in the same 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-1-2 router—A router with both Level- 1 and Level-2 router functions. It can establish Level- 1
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neighbor relationships with the Level- 1 and Level- 1 -2 routers in the same area, and establish Level-2
neighbor relationships with the 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.
Level- 1 routers in different areas cannot establish neighbor relationships. Whether Level-2 routers
establish neighbor relationships does not depend on areas.
Figure 29 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.
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