Right now, the approach that is followed will fetch all the dpns associated to the router, when the router is attached to a bgpvpn_internet. the changes handles, both the cases, where an external network is associated to a bgpvpn, and when a new router interface is added.
But, this will not handle a situation where, one boots a new VM on a new compute, after the router is associated to a bgpvpn_internet. On a VM boot, the router and its dpn is added to the NeutronRouterDpns data-store. If a routerDpns entry is not present when a new VPN is booted, then one will have to add the fallback rule for that VPN. Similarly, when the last interface for a router on a dpn is removed from the routerDpns data-store, one will need to remove the fallback rule.