Explanation: In AWS Direct Connect, your network must support Border Gateway Protocol (BGP) and BGP MD5 authentication, and you need to provide a private Autonomous System Number (ASN) for that to connect to Amazon Virtual Private Cloud (VPC). To connect to public AWS products such as Amazon EC2 and Amazon S3, you will also need to provide a public ASN that you own (preferred) or a private ASN. You have to configure BGP in the Create a Virtual Interface step.
Reference: http://docs.aws.amazon.com/directconnect/latest/UserGuide/getstarted.html#createvirtualinterface