Which advantage of placing the IS-IS Layer 2 flooding domain boundary at the core layer in a three-layer hierarchical network is true?

IS-IS L1 and L2 domains do not meaningfully overlap; L1/L2 boundary routers sit at the edge between the two domains, and placing the boundary at the core does not cause them to overlap.

When the L2 domain is confined to core routers, only core-layer topology changes trigger L2 LSP flooding and SPF recalculations across the backbone. Distribution and access layer events remain local to their L1 areas, preventing those changes from destabilizing the entire network, which makes the L2 domain both more contained and more stable.

Placing the L2 boundary at the core is best suited to a hierarchical hub-and-spoke topology and is not universally applicable to any kind of topology.

Constraining L2 to the core does not reduce L1 domain complexity; each L1 area retains its own full topology and LSP flooding scope regardless of where the L2 boundary sits.