Types of IS-IS Routers
Figure 4-1 shows an IS-IS network divided into areas. The IS-IS backbone is
not a specific area, as in OSPF, but an unbroken chain of routers doing Level
2 routing. R3, R6, and R4 are the backbone in Figure 4-1.
Within an area, routers can be one of three types:
�¡ Level 1 (L1) router.R1, R2, and R5 in the figure. Routes to
networks only within the local area (intra-area routing). Uses a default
route to the nearest Level 2 router for traffic bound outside the area.
Keeps one LSDB for the local area. When routing, compares the area
of the destination to its area. If they are the same, routes based on
system ID. If not, sends traffic to Level 1-2 router.
�¡ Level 2 (L2) router.R6 in the figure. Routes to networks in other
areas (interarea routing). The routing is based on area ID. Keeps one
LSDB for routing to other areas.
�¡ Level 1-2 (L1-2) router.R3 and R4 in this figure. Acts as a gateway
into and out of an area. Does Level 1 routing within the area and Level
2 routing between areas. Keeps two LSDB: one for the local area and
one for interarea routing.
The IS-IS method of selecting routes can result in suboptimal routing
between areas. To solve this, RFC 2966 introduces route leaking, which
allows some L2 routes to be advertised (or leaked) into L1 areas.