ANS-C01 Network Implementation Practice Question
Exhibit
Refer to the exhibit. ``` AWS Transit Gateway Route Table Route Table ID: tgw-rtb-0123456789abcdef0 Routes: 10.0.0.0/16 attachment tgw-attach-11111111111111111 (VPC-A) 10.1.0.0/16 attachment tgw-attach-22222222222222222 (VPC-B) 0.0.0.0/0 blackhole ``` ``` VPC-A Route Table (public subnet) Destination Target 10.0.0.0/16 local 10.1.0.0/16 tgw-1234567890abcdef0 0.0.0.0/0 igw-1234567890abcdef0 ```
A company has set up a transit gateway with attachments to VPC-A and VPC-B. The transit gateway route table shows routes to both VPCs and a blackhole for 0.0.0.0/0. VPC-A's public subnet route table sends 10.1.0.0/16 traffic to the transit gateway. However, an EC2 instance in VPC-A's public subnet cannot reach an instance in VPC-B. What is the most likely cause?
⚠ Common exam trap
AWS often tests the misconception that a transit gateway route table alone ensures bidirectional connectivity, when in fact each VPC's subnet route tables must have explicit routes for the other VPC's CIDR to enable return traffic.
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
✓
VPC-B's route table does not have a route to VPC-A's CIDR via the transit gateway.
For traffic to flow from VPC-A to VPC-B via a transit gateway, both VPCs must have routes in their route tables pointing to the transit gateway for the other VPC's CIDR. Since VPC-A's route table sends 10.1.0.0/16 (VPC-B's CIDR) to the transit gateway, but VPC-B's route table lacks a return route to VPC-A's CIDR via the transit gateway, the return traffic from VPC-B is dropped, causing connectivity failure.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
VPC-B's route table does not have a route to VPC-A's CIDR via the transit gateway.
Why this is correct
For bidirectional communication, VPC-B must also have a route back to VPC-A via the transit gateway.
- ✗
VPC-A's route table does not have a route to the transit gateway.
Why it's wrong here
It has a route to 10.1.0.0/16 via the transit gateway.
- ✗
The transit gateway route table does not have a route for 10.0.0.0/16.
Why it's wrong here
It does have a route for 10.0.0.0/16 via VPC-A attachment.
- ✗
The blackhole route in the transit gateway is blocking traffic between VPCs.
Why it's wrong here
Blackhole is for 0.0.0.0/0, not for specific VPC CIDRs.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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