code: def ec2_route_table_main_route_table_id(self, lookup, default=None):
      
      vpc_id = self.ec2_vpc_vpc_id(lookup)
      if vpc_id is None:
        return default
      route_table = EFAwsResolver.__CLIENTS["ec2"].describe_route_tables(Filters=[
        {'Name': 'vpc-id', 'Values': [vpc_id]},
        {'Name': 'association.main', 'Values': ['true']}
      ])
      if len(route_table["RouteTables"]) is not 1:
        return default
      return route_table["RouteTables"][0]["RouteTableId"]
docs: Args:
        lookup: the friendly name of the VPC whose main route table we are looking up
        default: the optional value to return if lookup failed; returns None if not set
      Returns:
        the ID of the main route table of the named VPC, or default if no match/multiple matches found