New VPC Subnets and Transit Gateway attachment

Question

Customer has an existing VPC with 2 subnets which are attached to a Transit Gateway. The subnet is running out of available IP space. So they added a new CIDR range to the VPC and added two subnets (Subnet 3 and 4). Subnet 1 and Subnet 3 share the same AZ while Subnet 4 is in a new AZ.

When attaching the new subnets to the Transit Gateway, you can select Subnet4 to be attached. Since Subnet 1 and Subnet 3 are in the same AZ, the Transit Gateway attachment option allows you to select only subnet 1 or subnet 3.

The question is whether Transit Gateway will know a route/path to subnet 3 if the transit gateway attachment is not explicitly made. In another word, would you need to detach subnet 1 from the Transit Gateway attachment and attach subnet 3?

Accepted Answer

You can only have 1 attachment per AZ. In fact, you need to have 1 attachment per AZ that you have created subnets in. The next step is to make sure the route table associated to each subnet has the required routes pointing to the Transit Gateway. These will be static routes.

On the TGW routing side, if you have route propagation enable with TGW route tables (or default route table) the entire added CIDR will be advertised automatically. if you do not have route propagation in the TGW, you can add the entire CIDR or what every subnets you want to advertise statically in the TGW.

New VPC Subnets and Transit Gateway attachment

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