Question for Transist Gateway CIDR Block

Transit Gateway CIDR blocks serve a specific purpose in AWS networking configurations. They are primarily used when setting up Connect (GRE) attachments or PrivateIP VPNs with your Transit Gateway. The Transit Gateway uses these CIDR blocks to assign IP addresses for tunnel endpoints in these types of connections.

If you're not seeing the CIDR blocks you've configured, it's important to note that modifications to a Transit Gateway, including CIDR block changes, only apply to new attachments. Existing attachments are not affected by these changes. This could explain why you don't see the CIDR blocks immediately after configuration.

In your production environment, where you can see the CIDR blocks, it's likely that they have been properly configured and are being used for Connect attachments or PrivateIP VPNs.

If you're unable to see or modify CIDR blocks in your test environment, there could be a few reasons:

1. You might not have the necessary permissions to view or modify these settings.
2. The Transit Gateway might have been shared with you from another account, in which case you cannot modify its configuration.
3. There might not be any Connect or PrivateIP VPN attachments that require these CIDR blocks.

To troubleshoot this issue:

1. Ensure you have the correct permissions to modify the Transit Gateway.
2. Verify that you're not working with a shared Transit Gateway.
3. Check if you have any Connect or PrivateIP VPN attachments that would utilize these CIDR blocks.
4. If you're certain you should be able to see and modify these settings, you may want to contact AWS support for further assistance.

Remember, Transit Gateway CIDR blocks are optional and are only necessary for specific types of attachments. If you're not using Connect or PrivateIP VPN attachments, you may not need to configure these CIDR blocks at all.
Sources
Get started with using Amazon VPC Transit Gateways - Amazon VPC
Modify a transit gateway using Amazon VPC Transit Gateways - Amazon VPC