A company is planning to migrate to AWS and use multiple VPCs in multiple AWS Regions. A network engineer must connect the eu-west-1 and eu-central-1…

The configuration is missing the necessary routes in the VPC route tables. For an instance in a VPC to send traffic to a remote network via a Transit Gateway, its subnet's route table must contain a route that directs that traffic to the Transit Gateway attachment. While the engineer configured routing within the Transit Gateways (steps 5 and 6), the initial step of getting traffic from the source VPC to its local Transit Gateway was omitted. Adding a route for the destination CIDR (or a summary route like 10.0.0.0/8 which covers both VPCs) in each VPC's route table with the target as the Transit Gateway resolves this connectivity issue.

A. Peering attachments are logical constructs and do not have configurable IP address ranges. This option is technically invalid.

B. Static routes are required in Transit Gateway route tables to direct traffic over a peering attachment. Dynamic propagation of VPC routes is not supported across peering.

D. Dynamic route propagation for VPC CIDRs is not supported across a Transit Gateway peering attachment. Static routes are mandatory for inter-region traffic.

1. AWS Documentation, Amazon VPC, User Guide, "Route tables for your VPCs", section "Routing". This document states: "Each subnet in your VPC must be associated with a route table. ... We recommend that you add a route to your subnet route tables for all traffic that is not destined for the VPC (0.0.0.0/0) that points to a gateway." In this scenario, the traffic to the other VPC is "not destined for the local VPC" and must be routed to the Transit Gateway.

2. AWS Documentation, AWS Transit Gateway, User Guide, "AWS Transit Gateway peering attachments", section "Routing for peering attachments". This document clarifies the routing requirements for peering: "To route traffic between the transit gateways, you must add a static route to your transit gateway route table that points to the transit gateway peering attachment." This confirms that step 6 in the scenario was correct and that options B and D, which suggest removing static routes, are incorrect.

3. AWS Documentation, AWS Transit Gateway, User Guide, "Examples", section "Centralized router". The diagrams and routing table examples in this section clearly show two distinct sets of routes required for connectivity: (1) routes within the VPC subnet route tables pointing to the Transit Gateway for remote CIDRs, and (2) routes within the Transit Gateway route table pointing to the appropriate attachments. The scenario is missing the first set of routes.