Hello @Searon ,
>> I know It's a bit off topic but for the connection between the local routers (brussel, paris) with the ISP would you also use static routes or bgp?
it is just a question of choice.
you can use eBGP on the branch routers using other two BGP ASes and configure additional neighbor commands on the appropriate routers ISP-x just as you have done within the ISP network block.
as an alternative you can have static routes on the ISP-x routers for the LAN subnets of the branch offices and then in BGP you add network commands for those subnets in the appropriate ISP-x routers.
Note: with BGP a network command applies not only to connected subnets but to every prefix in the IP routing table either connected , either via static either via dynamci routing protocol. So you just need network command with the correct mask (BGP looks for an exact match of the prefix and prefix length = subnet mask)
In this case the branch offices routers just need a default static route pointing to the direcly connected neighbor.
Again to be able to ping the internet the " internet" router will need additional static routes for the branch offices IP subnets.
Hope to help
Giuseppe
