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I saw AWS architecture that IGW is placed in front of ELB (i.e., see page 8 in http://www.slideshare.net/AmazonWebServices/high-availability-application-architectures-in-amazon-vpc-arc202-aws-reinvent-2013); and I also see ELB is placed directly to Internet connection (i.e., see page 14/15 at http://www.slideshare.net/AmazonWebServices/stg305)

What's the use cases for either architecture? or what's the pros and cons for either architecture design?

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IGW is implied on the second slides too, because otherwise your VPC won't be connected to the World.

An Internet gateway is a (..) VPC component that allows communication between instances in your VPC and the Internet. (from doc)

Update: Have just tested: IGW cannot be deleted until you have a route to 0.0.0.0/0 and a LB. It says 'Network vpc-* has some mapped public address(es). Please unmap those public address(es) before detaching the gateway'

After deleting the dependencies I was able to delete the IGW, but when then I re-created the LB it was assigned with internal IP only.

Just as a note, you can create an ELB only within a VPC, it means the LB will be inside it, thus needs a route and gateway to get outside.

  • I think ELB is good enough for forwarding HTTP request to the backend servers, so that even the backend servers are not internet routable, they can handle http requests. Therefore, I don't think IGW is required in this case – chen Jun 1 '16 at 16:25
  • @chen please review the answer, checked and updated. – Putnik Jun 2 '16 at 10:35

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