Routing (BGP) of location with two active VPNs to hubs that exchange routing info

Hi all

This might be an odd question and I might just be too dense to see the solution. It is not excactly a Fortinet specific question (its about BGP, routing) - however, since it is realised with Fortigates, I thought I might ask here as well

The situation is as follows:

• I have >20 locations in a country which has two "country" hubs.

• Every location has an active ipsec tunnel to each of the hubs (meaning: two active vpn connections)

• One tunnel will be considered "primary" handling all the traffic, the other will considered "seconday" and should only handle traffic when the primary is down.

• The choice of primary and secondary tunnel or hub is made on location basis (meaning both hubs have primary and secondary connections)

• With every ipsec tunnel they propagate their network to the hub(s).

• So every hub has the information of each connected location

• The hubs are also connected with each other

• The hubs act as route reflector

• The hubs also have connections to other countries and propagate the learned networks from the locations to those countries (and vice versa).

Now - how can I make sure that the traffic only goes via the primary ipsec tunnel (unless down and the secondary takes over) when everyone learns every route/network via both tunnels?
How can I avoide an asynchrounous routing? (I guess that will happen at some point if I do nothing).

Thanks a lot for your input