The point is, you cannot rely on TTL. TTL could be hard coded on a server to 5 minutes or to 20 days. Doesn't need to be a BFE server, it could be a Comcast DNS

The point is, you cannot rely on TTL. TTL could be hard coded on a server to 5 minutes or to 20 days. Doesn't need to be a BFE server, it could be a Comcast DNS for all you're aware. This is why DNS "failover" is never a good idea outside of a highly controlled (i.e., internal) environment.