Consider the exhibit. All routers are running IS-IS with IPv6 support enabled. Based on the topology shown, and the route tables of routers R3 and R4, which of the following statements is TRUE?

R3 is a Level 1 router in Area 1, and R4 is a Level 1 router in Area 2. By default, a Level 1 router only learns routes within its own area and receives a default route from the nearest L1/L2 router (ABR) to reach other areas. The route tables show that R3 has a specific route to R4's loopback (in Area 2) and R4 has a specific route to R3's loopback (in Area 1). The presence of these specific inter-area prefixes in the routing tables of L1-only routers is direct evidence of L2-to-L1 route leaking. This leaking is configured on the ABRs (R1 and R2). For redundancy and consistent routing, this policy would be configured on both ABRs.

A. The presence of specific inter-area routes in the L1 routers' tables explicitly proves that route leaking is configured.
B. This is unlikely. If only R1 were leaking, R3 and R4 might have suboptimal paths via R1, and the configuration would lack redundancy.
C. This is unlikely for the same reason as option B; it would create an asymmetric and non-redundant routing policy.

Nokia (2021), SR OS 21.10.R1 Router Configuration Guide, "IS-IS Route Leaking". This section describes how L2 routes can be leaked into an L1 area using route policies on an L1/L2 router. It explains that this provides more specific routing information than the default route generated by the Attached-bit (ATT-bit).

RFC 1195, Use of OSI IS-IS for routing in TCP/IP and dual environments, Section 4.2.3 "Inter-Area Routing". This RFC describes the default behavior where L1 routers use an attached L1/L2 router to reach other areas. Route leaking is the mechanism to override this default and provide specific inter-area prefixes.