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TransitGatewayAttachments list in CLI response can have multiple attachments if you have multiple VPN connections with with BGP and ECMP is enabled. Then it would list all of the VPN connection attachments there.
Here's an example what an entry would look like the scenario mentioned above.
"Routes": [ { "DestinationCidrBlock": "10.0.0.0/24", "TransitGatewayRouteTableAnnouncementId": "", "TransitGatewayAttachments": [ { "ResourceId": "vpn-XXXXXXXXXX(xx.xx.xx.xx)", "TransitGatewayAttachmentId": "tgw-attach-XXXXXXXXXX", "ResourceType": "vpn" }, { "ResourceId": "vpn-YYYYYYYYYY(yy.yy.yy.yy)", "TransitGatewayAttachmentId": "tgw-attach-YYYYYYYYYY", "ResourceType": "vpn" } ], "Type": "propagated", "State": "active" }
0
I can't see a case where multiple attachments come under same destination CIDR block. I think it's a List to neatly handle the fact that there isn't always one attachment - there can zero or one. A Blackhole route will have zero attachments.
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Also, if you try creating a route with same destination CIDR, it throws an error: "Route <CIDR> already exists in Transit Gateway Route Table <table-ID>"
You are referring to V1 documentation of the CLI, are using V1 of the CLI or V2 which is the latest? It has up to date documentation located at: https://awscli.amazonaws.com/v2/documentation/api/latest/reference/ec2/search-transit-gateway-routes.html