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  • Hirofumi Takahashi thinks this is interesting:

Area ID 0 (or 0.0.0.0) is reserved for the backbone. The backbone is responsible for summarizing the topologies of each area to every other area. For this reason, all inter-area traffic must pass through the backbone; non-backbone areas cannot exchange packets directly.

From

Cover of Routing TCP/IP, Volume 1, 2/e

Note

Backbone Area