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  • Eiji Tanioka thinks this is interesting:

Before Phase 3, spoke-to-spoke tunnels were established through regional hubs. In Phase 3, spoke-to-spoke tunnels are established directly with each other, bypassing both the regional and central hubs.

From

Cover of CCIE Professional Development Series Network Security Technologies and Solutions

Note

Phase2 is for single-region design, so spoke-to-spoke tunnel between region needs "HUB-TO-HUB' connection.