Objectives
The simulator ticks each one as soon as the network meets it.
R1: NET 49.0001.0000.0000.0001.00, Level-1 only.
R2: NET 49.0001.0000.0000.0002.00, Level-1-2 (the default level).
R3: NET 49.0002.0000.0000.0003.00, Level-2 only.
Enable IS-IS on the links; make Loopback0, R1 G0/0 and R3 G0/1 passive.
Check show isis neighbors on R2: R1 is an L1 neighbor and R3 an L2 neighbor. R1 has no route to 192.168.3.0/24, only an IS-IS default route via R2.
PC1 pings PC3 (192.168.3.10).
Before you start
Level-1 adjacencies need the same area; Level-2 adjacencies do not. An L1/L2 router copies Level-1 routes into Level-2 but not the reverse, and sets the ATT bit when it reaches other areas, so Level-1 routers install a default route toward it. On Cisco the default is-type is level-1-2 and IS-IS routes have distance 115.
Concept
IS-IS levels and the ATT default route
IS-IS is the link-state protocol most providers run in their core. Routers are identified by a NET such as 49.0001.0000.0000.0001.00 (area, system ID, selector). Level 1 routes within an area, Level 2 between areas, and Level 1-2 routers join the two.
