When you design a network, when would it be required to leak routes into a Level 1 area?

Multicast RP reachability does not require route leaking because PIM can resolve the RP address using the default route that Level 1 routers already receive from the nearest L1/L2 boundary router.

MPLS L3VPN PE routers in a Level 1 area must have specific routes to remote PE devices to signal Label Switched Paths via LDP or RSVP-TE and to perform CEF-based VPN forwarding decisions. A default route toward the L1/L2 boundary router is insufficient for end-to-end LSP establishment, making Level 2 to Level 1 route leaking mandatory in this scenario.

Equal cost load balancing between backbone and nonbackbone areas is handled natively by the default route mechanism in Level 1 and does not require specific route leaking from Level 2.

Unequal cost load balancing is not a recognized technical justification for IS-IS route leaking; Level 1 routers forward traffic toward the closest L1/L2 router regardless of metric variation to the destination.