This is related to a similar question IS-IS multi area But instead of the how is the destination router chosen, I want to know how L1 routers choose the first L1/L2 router.
Using the below topology:
R1 and R6 are L1 only
R2, R3, R4, R5 are L1/L2
which Router would R1 choose to use when pathing to R6? R3 provides the shortest total path (50), but R2 is the closer L2 capable router (70).
In my own testing but with different costs, I have seen R1 take the closer L2 router if both paths are the same total cost, and I have seen R1 take the other if the closer one has a longer path.