Colin
If i understand correctly you are going to have each DC advertise out it's own subnets but not each others. If this is the case then yes you can do it but you need extra configuration. The issue is that the AS PATH is used for loop prevention so if an EBGP router receives a route advertisement and sees it's own AS in the path it will not accept the route. So on your DC BGP routers you would need to add -
router bgp
neighbor allowas-in where x is the number of occurences of your own AS in the path.
Obviously because you are overriding the loop protection measure you need to be sure you are not creating a routing loop but from what you have described that is highly unlikely.
In fact this is quite a common setup with an MPLS VPN setup where a customer uses the same AS for multiple sites and peers with the provider using their own AS. I appreciate this is not MPLS but the principle is the same.
With the above configuration each DC will receive and accept the route advertisements for the other DC via the provider connections and should be able to route between each other.
How are you proposing to use your secondary connections. Is it just for backup in case the primary link fails or are you looking to load share at each DC ?
Jon
