I have a router that takes a long time to converge after reboot. To fix that I do not want to advertise my prefixes until the router is fully ready. But I still want to establish the BGP sessions otherwise the router will never be ready. So we program in a delay until advertising after BGP session established. Now if my peers automatically converted BGP announced prefixes into ACLs, they would blackhole any traffic that might come to this router during startup. This is obviously not good. BGP announced prefixes tells you what I can receive but not what I can send. Interpreting that any other way is abusing the protocol. You would need a new BGP extension so we could announce what we might send independent of what we want to receive. IRR generated ACL filters might work if agreeable by the peer. Regards Baldur