def get_database_subnet_group_description(session, identifier):
    """
    Returns database subnet_group description document or None.
    Args:
        session (:class:`boto.rds2.layer1.RDSConnection`): The RDS api
            connection where the database is located.
        identifier (str): The RDS database instance identifier.
    Returns:
        dictionary: description of RDS database instance
    """
    try:
        return session.describe_db_subnet_groups(DBSubnetGroupName=identifier)[
            "DBSubnetGroups"
        ][0]
    except session.exceptions.DBSubnetGroupNotFoundFault:
        return None