The best way they can do this is to filter their announcements such that they will only hear certain specific routes from their customers. You may remember major network problems from late April that could have been avoided if any of a number of routers had strict filters installed. That host has to have a path back to you. Look at Figure 1. Similarly, if there is not a generally advertised route to cover an IP address, then no one on the Internet will be able to reach it.
All filters have an implicit deny .
Whether it's a BGP route or a static internal route, the most specific route always wins. Once BGP picks the best route, it is then eligible to be installed in the IP Routing Table, which is the table that the router consults when making the actual packet-forwarding decisions. For the example in this column we'll explicitly advertise only a few routes, and use AS-PATH filters to deny advertisement of any other BGP routes we may have heard. Ideally, you'd like roughly half the traffic to go in and out of each connection. In an ideal network, you'd be able to have any one of your connections to the Net go down and still maintain connectivity and speed.