Using EBGP between regions with intra-region ADVPN

A straightforward approach is to use EBGP between the regional gateways. With EBGP, each gateway advertises a summary route of all regional prefixes to all remote regions. Those will, in turn, advertise default routes to their branches. A branch willing to communicate to a remote region will always send traffic to its local, regional gateway, which will use the correct summary route to forward the traffic to the remote regional gateway.

ADVPN will be used only for branch-to-branch traffic within each region, while the traffic across the regions will always flow via the regional hubs.