**To modify a transit gateway VPC attachment** The following ``modify-transit-gateway-vpc-attachment`` example adds a subnet to the specified transit gateway VPC attachment. :: aws ec2 modify-transit-gateway-vpc-attachment \ --transit-gateway-attachment-id tgw-attach-09fbd47ddfEXAMPLE \ --add-subnet-ids subnet-0e51f45802EXAMPLE Output:: { "TransitGatewayVpcAttachment": { "TransitGatewayAttachmentId": "tgw-attach-09fbd47ddfEXAMPLE", "TransitGatewayId": "tgw-0560315ccfEXAMPLE", "VpcId": "vpc-5eccc927", "VpcOwnerId": "111122223333", "State": "modifying", "SubnetIds": [ "subnet-0e51f45802EXAMPLE", "subnet-1EXAMPLE" ], "CreationTime": "2019-08-08T16:47:38.000Z", "Options": { "DnsSupport": "enable", "Ipv6Support": "disable" } } } For more information, see `Transit gateway attachments to a VPC <https://docs.aws.amazon.com/vpc/latest/tgw/tgw-vpc-attachments.html>`__ in the *Transit Gateways Guide*.