Is-Is Area - HPE 5800 Series Configuration Manual

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 two-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 a 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 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 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 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.
Level-1 routers in different areas cannot establish neighbor relationships. Whether Level-2 routers
establish neighbor relationships does not depend on areas.
Figure 36 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.
Figure 36 IS-IS topology 1
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