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Configuring IS-IS IS-IS Overview
OmniSwitch 6800/6850/9000 Advanced Routing Configuration Guide December 2007 page 3-11
An area’s topology is visible only to the members of that area. Routers inside a given area do not know the
detailed topology outside the area. This isolation of knowledge enables the protocol to reduce routing traf-
fic by concentrating on small areas of an AS, as compared to treating the entire AS as a single link state
domain. In IS-IS, the router belongs entirely to a single area.
When an AS is split into IS-IS areas, the routers are classified into the following three categories:
• Level-1 routers—These are Intra-area routers and form relationship with other Level-1 or Level-1/2
routers within the same area.
• Level-1/2 routers—These routers form relationship with other Level-1, Level-2, or Level-1/2 routers.
They are used to connect Inter-area routers with Intra-area routers.
• Level-2 routers—These are Inter-area routers and form relationship with other Level-2 or Level-1/2
routers
These Level capabilities can be defined globally on a router or on specific interfaces. Since a separate
copy of the link state algorithm is run in each area, most configuration parameters are defined on a per-
router basis. All routers belonging to an area must agree on that area’s configuration. Misconfiguration
will keep neighbors from forming adjacencies between themselves, and IS-IS will not function.