AWS Certified Solutions Architect - Professional 2020

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Transitive VPCs


The question from quiz:

You have setup VPC Peering between VPC_BLUE and VPC_YELLOW and also VPC_BLUE and VPC_RED. In testing, you realize that you cannot reach VPC_RED from VPC_YELLOW. What is the simplest way you can fix this?

Doesn’t the answer ‘Setup a peering connection between VPC_YELLOW and VPC_RED.’  imply that VPCs were transitive?



Deepak Adlakha

What are the options in the Questions ?

1 Answers

This is your starting point:

YELLOW <---> BLUE <---> RED

BLUE can talk to YELLOW and RED, but RED cannot talk to YELLOW because it should transit over BLUE in order to reach YELLOW. And that is transitive routing, which is not allowed.

Then, the solution is this:

YELLOW <---> BLUE <---> RED
^                        ^

Then you have a VPC peering between RED and YELLOW, and now they can talk to each other over the new peering. There is no need to traverse over BLUE.

It is also a full mesh, meaning that every VPC has a peering to the oter VPCs. Which is not always feasible because peerings: (N*(N-1)/2) N=3 –> 3 (Imagine that for 10 VPCs we would need 45 peerings, hard to manage)

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