IS-IS

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Intermediate-System to Intermediate-System

IS-IS is a link state Routing protocol which are also used in IP network. Here it is called integrated IS-IS. IS-IS is often used by providers as an IGP.

Level 1 Routing

A level 1 Router only knows about the area in which it is located. Like a stub Router in OSPF. It has no knowledge of other areas. A level 1 Router only knows a default Route to the nearest level 2 Router if Routing between areas is necessary. Every Level 1 Router in that area has the same link State Database containing information about that area.
Level 1 Routers are referred to as Intra-area Routers

Level 2 Routing

A Level 2 Router routes between areas and are referred to as Backbone Routers. The backbone must be contiguous because all Level 2 Routers share the same Link-State Database which contain prefixes from all areas. If the backbone is fractured Routing information will not be the same on all Level 2 Routers.
Level 2 Routers are referred to as Inter-area Routers

Level 1-2 Routers

The Level 1-2 Router has both the Level 1 Link-state database containing information about the area in which it participates and Level 2 Link-state database containing information about all areas.

Areas and backbone Routers example
A contiguous IS-IS Backbone example