AWS Transit Gateway isolated routing with Shared Services

Question

Hello,

We are working with a client that has the following setup:
1 TGW that has 5 VPC attachments

- non-prod a attachment
 - non-prod b attachment
 - prod-a attachment
 - prod-b attachment
 - shared-services attachment

**Requirements:**
The client would like to allow only for inter-communication of both the non-prod VPCs, as well as the prod VPCs. Additionally, the client would like for shared-services to be able to reach any of the VPCs.

**My initial approach and the problem:**
Initially, my thought was to create 3 isolated routing domains, which were represented by 3 separate TGW route tables. I was unable to achieve this because there is a limitation of only being able to associate 1 attachment with 1 TGW route table. If we were able to make more than one attachment for the same VPC, we could get around this issue, but unfortunately that is not possible.

**Looking for recommendations on the best way to address the above requirements.**

The only other option I can think of to accomplish these requirements is to use 1 TGW route table, associate all of the attachments with it, and then ultimately use NACLs in the individual VPCs to restrict traffic. To me, this seems like a pain to maintain, so I wanted to see if there are any other ways to address this problem.

----------

**More Information on what I've already tried**

My initial idea was to have 3 TGW route tables (one for each "routing domain"):

- non-prod (associated with the non-prod-* attachments)
 - prod (associated with the prod-* attachments)
 - shared (associated with all attachments)

The problem with this approach is the **shared** TGW route table because as I mentioned above, you can't associate an attachment with more than one route table. In essence this is a "flat" routing architecture rather than isolated and directly contradicts with what I'm trying to do causing this problem.

Thanks in advance!

Accepted Answer

I think this gets what you need?

**NPA and NPB attachment Route Table:**

NPA CIDR --> NPA attachment
    NPB CIDR --> NPB attachment
    SS CIDR --> SS attachment

Traffic arriving from NPA or NPB attachment can go SS, NPA, or NPB

**PRA and PRB attachment route table**

PRA CIDR --> PRA attachment
    PRB CIDR --> PRB attachment
    SS CIDR --> SS attachment

Traffic arriving from PRA or PRB attachment can go SS, PRA or PRB

**SS Attachment route table**

NPA CIDR --> NPA attachment
    NPB CIDR --> NPB attachment
    PRA CIDR --> PRA attachment
    PRB CIDR --> PRB attachment

Traffic from SS attachment can go NPA, NPB, PRA, or PRB

AWS Transit Gateway isolated routing with Shared Services

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