cant route traffic through transit gateway.

Question

Hi, 
I have a transit gateway with 4 VPC attached. VPC A, B, C, and D. The requirement is that traffic is allowed between VPC A, B, and C. Traffic is allowed between VPC B and D. Traffic is not allowed between A, C, and D. So I created two separate route tables RTB1 and RTB2. RTB1 has an association with A, B, and C and propagation of the same too. RTB2 has an association with B and D and propagation of the same B and D. The first route works fine. Traffic is working between A, B, and C. The second route is not working. Traffic is not working between D & B. Am I missing something? All help is appreciated. I have checked the security group and VPC route tables also. Everything looks fine.
Thanks

Oleksii Bebych · Accepted Answer

Separate TGW Route Table for each TGW attachment

* A can connect to B 
* A can connect to C
* B can connect to A
* B can connect to C
* B can connect to D
* C can connect to A
* C can connect to B
* D can connect to B

![Enter image description here](/media/postImages/original/IM5oRexM81QhqS_y3UCVPgKQ)

Oleksii Bebych · Answer

> RTB1 has an association with A, B, and C and propagation of the same too.

so you don't have route from B to D

> RTB2 has an association with B and D and propagation of the same B and D

This is not possible. Because TGW Attachment can be associated only with one  TGW Route Table

If you try to associate the second one, you will get an error:
```
There was an error creating your transit gateway route table association.
Transit Gateway Attachment tgw-attach-0fc***** is already associated to a route table.
```

The simplest way for you would be to have a separate TGW Route table for each TGW attachment and each Route Table would have only the necessary routes

aws_newbee · Answer

My apology. RTB2 has only an association of D but the propagation of B and D.

Gary Mclean · Answer

Have you added the routes in the route tables of B and D to route traffic back to the TGW attachment?

cant route traffic through transit gateway.

Relevant content