When is it required to leak routes into an IS-IS Level 1 area?

IS-IS does not support unequal cost load balancing natively - this is an EIGRP-specific feature and is not a valid use case that would require route leaking into a Level 1 area.

A multicast Rendezvous Point configured in a non-backbone area does not inherently require IS-IS route leaking, as the RP address is distributed through multicast control protocols such as PIM or BSR rather than through IGP route leaking.

Equal cost load balancing between backbone and non-backbone areas can function using the default Level 1 behavior with the attached-bit generating a default route; specific route leaking is not required solely to achieve ECMP in this scenario.

MPLS L3VPN PE devices require end-to-end IP reachability between their loopback interfaces to establish LDP or RSVP-TE sessions and exchange VPN routes via MP-BGP. Level 1 areas by default only install a default route toward the Level 2 backbone, which is insufficient for establishing targeted LDP sessions or resolving specific PE loopbacks in other areas. Route leaking redistributes specific Level 2 prefixes into the Level 1 area so that PE devices can resolve exact next-hops required for MPLS label forwarding.