Review Set 2 – VPC peering cfgs – route to specific addresses.

[AudreyST]

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CSAP: Design for New Solutions. Q: “A company has three AWS accounts each with its own VPCs as shown in the diagram. VPC-B and VPC-C have matching CIDR blocks …Which… allow VPC-A to communicate with the database instance in VPC-B while being able to communicate with all resources on VPC-C?” (See 1st diagram).

A: “On VPC-A, add a static route for VPC-B CIDR (10.0.0.77/32) with the target pcx-aaaabbbb and another static route for VPC-C CIDR (10.0.0.0/16) with the target pcx-aaaacccc. On VPC-B, add a static route for VPC-A CIDR (172.16.0.0/24) with the target pcx-aaaabbbb. On VPC-C, add a static route for VPC-A CIDR (172.16.0.0/24) with the target pcx-aaaacccc.”

However, In the explanation section, the routes from VPC-B & VPC-C to VPC-A are 172.16.0.0/16 pcx-aaaabbbb & 172.16.0.0/16 pcx-aaaacccc, respectively, difference from the /24 in the answer (See 2nd diagram attached). A similar example in AWS doc matches the explanation (using /16). Is there a typo in the answer? Is 172.16.0.0/24 or 172.16.0.0/16 the correct answer?

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[Neil-TutorialsDojo]

Hello AudreyST,

Good day!

Thank you for your question.

The /16 CIDR block represents the entire address range for the VPC. Using /16
in the route table ensures that all IP addresses within VPC-A are reachable from VPC-B and VPC-C. In contrast, the /24 CIDR block used for Subnet-A specifies a smaller, specific address range within VPC-A.

Both approaches for routing are correct based on the given scenario, and the choice depends on whether you need to target specific subnets or the entire VPC.

I understand the confusion and apologize for any misunderstanding this may have caused. We will update the question as soon as possible.

Thank you for bringing this to our attention!

Regards,
Neil @ tutorials dojo

[Neil-TutorialsDojo]

Hello AudreyST,
This question has been updated. It should be reflected to the portal soon. Thank you