When a Level 1/Level 2 router (Router B) finds out 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 1 LSP. Thereafter, the Level 1 router (Router A) introduces a default route pointing to the nearest attached Level 1/Level 2 router (Router B). See Figure 7.
Figure 7: Install Default Route to Nearest Level 1/Level 2 Router
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