In Multi Area IS-IS how does an L1 router choose the L1/L2 to use for interarea routing?

Question

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.

Answer 1

L1 routers don't know anything about the inter-area topology, so they choose the nearest L2 router. R1 doesn't know about R6, the route is just "outside my area", so for R1 that's R2.

R2 doesn't know about R6 but it knows that it's inside area 2. The shortest path to area 2 is over R3. R3 routes to R5 which does know about R6 but uses the shorter path over R4.

Level 1 Intermediate Systems: These systems deliver and receive NPDUs from other systems, and relay NPDUs from other source systems to other destination systems. They route directly to systems within their own area, and route towards a level 2 Intermediate system when the destination system is in a different area.

ISO/IEC 10589

Rule of thumb for out-of-area:

• L1 routers use the nearest L2 neighbor as gateway
• L2 routers use the nearest L2 neighbor of destination area as gateway

Answer 2

A Level 1/Level 2 system sets the attached bit in the Level 1 PDUs that it generates into a Level 1 area to indicate that it is a Level 2-attached backbone router and that it can be used to reach prefixes outside the Level 1 area. Level 1 routers create a default route for interarea prefixes, which points to the closest (in terms of metrics) Level1/Level 2-attached router.

-Advanced Junos Service Provider Routing, Ch 5.