Question

A network engineer is creating a Transit Gateway VPC attachment for a VPC that spans three Availability Zones (us-east-1a, us-east-1b, us-east-1c). The VPC contains multiple workload subnets in each AZ. The engineer specifies only one subnet in us-east-1a and one subnet in us-east-1b when creating the attachment. After the attachment becomes available, the team reports that instances in us-east-1c cannot reach resources across the Transit Gateway. What is the cause and the correct fix?

Accepted Answer

The Transit Gateway attachment does not place an ENI in us-east-1c, so traffic originating there has no local attachment appliance; add a subnet in us-east-1c to the attachment so an ENI is provisioned in that AZ.

Answer

A Transit Gateway attachment automatically routes for all AZs regardless of the subnets chosen; the issue is a missing route in the us-east-1c subnet route table pointing to the Transit Gateway.

Answer

You must select every subnet in each AZ for the attachment; add all us-east-1c subnets, not just one, so each subnet gets its own Transit Gateway ENI.

Answer

Transit Gateway attachments require appliance mode to be enabled for multi-AZ VPCs; enabling appliance mode will make us-east-1c reachable.