ANS-C01 Network Design Practice Question
A company has deployed a multi-tier application across three VPCs (VPC-A, VPC-B, VPC-C) in the us-east-1 region. Each VPC has its own CIDR block (10.0.0.0/16, 10.1.0.0/16, 10.2.0.0/16 respectively). All VPCs are attached to an AWS Transit Gateway. The web tier is in VPC-A, the application tier is in VPC-B, and the database tier is in VPC-C. The application servers in VPC-B need to connect to the database servers in VPC-C on TCP port 3306. The database servers are in a private subnet with a security group (sg-db) that allows inbound traffic from the application server security group (sg-app). The application servers have a security group (sg-app) that allows outbound traffic to the database servers. However, the application servers cannot connect to the database servers. The route tables in VPC-B and VPC-C have routes pointing to the Transit Gateway for the other VPC CIDRs. The Transit Gateway has attachments in all three VPCs and has a default route table with propagation enabled. What is the MOST likely cause of the connectivity failure?
⚠ Common exam trap
Test-takers frequently assume enabling propagation on the default route table automatically allows all inter-VPC traffic, but they overlook the requirement that all attachments must be associated with the same route table for transitive routing to work.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The Transit Gateway attachments are not associated with the same route table
The most likely cause is that the Transit Gateway attachments are not associated with the same route table. In AWS Transit Gateway, for traffic to flow between VPCs, the attachments must be associated with a route table that contains routes for the destination VPC CIDRs. Even though propagation is enabled on the default route table, if the attachments are associated with different route tables (or the default route table is not shared), transitive routing will fail. Without a common route table association, the Transit Gateway cannot forward traffic between VPC-B and VPC-C.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
The Transit Gateway attachments are not associated with the same route table
Why this is correct
Correct; without association, routes are not propagated to the attachments.
- ✗
The network ACLs in VPC-B or VPC-C are blocking the traffic
Why it's wrong here
Incorrect; default NACLs allow all traffic, and no custom NACLs are mentioned.
- ✗
The Transit Gateway does not support transitive routing between VPCs
Why it's wrong here
Incorrect; Transit Gateway does support transitive routing.
- ✗
The security group sg-app does not allow outbound traffic to the database server's security group
Why it's wrong here
Incorrect; the scenario states that sg-app allows outbound to the database servers.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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