How many route table updates will I have to manage for large Transit Gateway deployments?
The AWS Transit Gateway (TGW) was introduced to eliminate complexity involved in peering many VPCs together. In the pre-AWS Transit Gateway (TGW) world, if you had to connect many VPCs, you were required to use a complex mesh of VPC peerings. To be accurate n (n-1)/2, where n is the number of VPCs.
AWS Transit Gateway (TGW) was introduced to make peering of VPCs easier. But, it does not make routing easy. When attaching the AWS Transit Gateway (TGW) to a VPC, it does not propagate routes to the VPCs. Similarly, creating propagation across route tables in the AWS Transit Gateway (TGW) does not automatically populate routes into the respective tables.
The formula of how many route tables need to be managed, reviewed and updated when using an AWS Transit Gateway (TGW) is equal to: n ( s + r - 1), where n is the number of VPCs attached to the AWS Transit Gateway (TGW), s is the number of subnets in each VPC and r is the number of route tables in the AWS Transit Gate