Transit Gateway - Disable Automatic Propagation

Question

Hi everyone,

I have a scenario where multiple VPCs, each with a single CIDR range and 3 subnets, are attached to a Transit Gateway, allowing full communication between all VPCs (something similar to the attached diagram).

![Enter image description here](/media/postImages/original/IMRfyYpMbeRQuxxTeF-_BbXA)

Now, I want to add a secondary CIDR let's say to VPC C to create additional subnets, but I need these new subnets to only be accessible within VPC C and not announced to the other VPCs connected via the Transit Gateway.

What would be the best way to configure this so that the new subnets remain local to VPC C while keeping the existing communication between the other VPCs intact given that the Transit Gateway is using a single Routing Table ?

Thanks in advance!

Leo K · Accepted Answer

The point of propagating routes from a VPC to a TGW route table is that all the CIDRs configured for the attached VPCs would become known to and reachable from the TGW route table. When your requirement seems to be effectively the exact opposite, manually choosing a single CIDR for each VPC to be reachable from other networks attached to the TGW, I would think the simplest solution would be to disable the automated route propagation and configure a route manually for each VPC's preferred CIDR in the TGW route table.

Disabling route propagation: https://docs.aws.amazon.com/vpc/latest/tgw/disable-tgw-route-propagation.html

Adding the preferred routes manually: https://docs.aws.amazon.com/vpc/latest/tgw/tgw-create-static-route.html

Disabling route propagation:
https://docs.aws.amazon.com/vpc/latest/tgw/disable-tgw-route-propagation.html

Adding the preferred routes manually:
https://docs.aws.amazon.com/vpc/latest/tgw/tgw-create-static-route.html

Transit Gateway - Disable Automatic Propagation

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