This is a very basic lab that currently runs IS-IS Level 1 in a single area. An interest quirk to note is that R1 has had ip router isis configured under its loopback, whereas R2 and R3 have had passive-interface loopback0 configured under the IS-IS process. The effect of this is that the metric to reach loopback0 on R1 will be higher as reaching the loopback is considered an extra ‘hop’ or link. Also, level-2 has not been explicitly disabled under the IS-IS processes anywhere in this lab, so default leaking of IP entries (but not IS entries) in the LSPs from L1 to L2 is taking place.
Devices: 3 x Cisco 3725