What are two characteristics of IS-IS?

IS-IS (Intermediate System to Intermediate System) was designed by ISO as a link-state Interior Gateway Protocol for routing within a single autonomous system, functioning on a hierarchical two-level model conceptually similar to OSPF's areas, which makes the first statement unambiguously correct. Within that hierarchy, Level 1 routers maintain detailed topology information only for their own local area and use a default route toward the nearest Level 2 or Level 1/Level 2 router to reach destinations outside the area; they explicitly do not carry inter-area topology information and therefore cannot route traffic between areas on their own. Routers that participate in both levels simultaneously — Level 1/Level 2 (L1/L2) routers — sit at the area boundary, maintaining a Level 1 database for their local area and a Level 2 database for the backbone, and it is specifically these dual-level routers that stitch inter-area traffic together, forwarding packets from the local area into the Level 2 backbone and vice versa, which confirms the second correct statement. Regarding route preference, Junos assigns distinct default preference (administrative distance) values of 15 for IS-IS Level 1 routes and 18 for IS-IS Level 2 routes — neither of which is 20 — so the preference-value statement is factually incorrect and included as a distractor testing precise knowledge of Junos default protocol preferences. Reference topics: Junos Enterprise Routing – IS-IS, Level 1, Level 2, and L1/L2 Router Roles; Default Route Preference Values.