Installing a Default Route to the Nearest Routing Device That Operates at Both IS-IS Levels

When a routing device that operates as both a Level 1 and Level 2 router (Router B) determines that it can reach at least one area other than its own (for example, in Area Y), it sets the ATTACHED bit in its Level  LSP. Thereafter, the Level 1 router (Router A) introduces a default route pointing to the nearest attached routing device that operates as both a Level 1 and Level 2 router (Router B). See Figure 7.