User guide

Configuring IS-IS IS-IS Overview
OmniSwitch AOS Release 7 Advanced Routing Configuration Guide March 2015 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
traffic 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.