That's not possible to control from the spoke side... That's the conclusion I'm coming to. The minute a unique prefix is advertised from one DC, spokes could end up taking the backup hub connection. And because our DCs exchange routes, DC "return" traffic prefers the spoke's primary AutoVPN connection due to MX prepending. Not a big deal right now since our DCs are the only egress points for internet traffic (did I mention this is a SDWAN MPLS deployment) so we simply advertise the default route to branch offices. Later on when we setup local internet egress, we'll need to advertise the same RFC1918 prefixes to spokes and let the DC COREs forward traffic accordingly. It's not idea since we'd prefer branch traffic to go directly to the respective DC but I don't see an alternative setup.
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