IPv6 Route in Route Table (rtb-xxx) with destination (::/0) already exists about terraform-aws-vpc HOT 9 CLOSED

Ok I believe I've found the root cause of the problem.
In my scenario I'm trying to deploy a VPC with 3 public, 3 private and 3 database subnets, furthermore I opted for having a single NAT gateway by setting "enable_nat_gateway = true" and "single_nat_gateway = true".

Performing some tests I've noticed that the terraform plan is trying to create 3 different "aws_route.private_ipv6_egress" routes, but the problem is that since only a single NAT Gateway is created (hence a single Route Table as you can notice from this comment the 3 created routes won't of course fit in the single Route Table.

This line is where supposedly each egress_ipv6 route should be associated with a different Route Table, but of course the count of the block will be equal to three while the aws_route_table.private[] will only have index 0.

To confirm my ipothesis I've tried to re-deploy the module using the same configuration but deploying one NAT Gateway per subnet ("enable_nat_gateway = true", "one_nat_gateway_per_az = false" and "single_nat_gateway = false") instead of a single one... and it worked.

Let me know if in your opinion I got something wrong

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Ok I believe I've found the root cause of the problem.
In my scenario I'm trying to deploy a VPC with 3 public, 3 private and 3 database subnets, furthermore I opted for having a single NAT gateway by setting "enable_nat_gateway = true" and "single_nat_gateway = true".

Performing some tests I've noticed that the terraform plan is trying to create 3 different "aws_route.private_ipv6_egress" routes, but the problem is that since only a single NAT Gateway is created (hence a single Route Table as you can notice from this comment the 3 created routes won't of course fit in the single Route Table.

This line is where supposedly each egress_ipv6 route should be associated with a different Route Table, but of course the count of the block will be equal to three while the aws_route_table.private[] will only have index 0.

To confirm my ipothesis I've tried to re-deploy the module using the same configuration but deploying one NAT Gateway per subnet ("enable_nat_gateway = true", "one_nat_gateway_per_az = false" and "single_nat_gateway = false") instead of a single one... and it worked.

Let me know if in your opinion I got something wrong

In my case I solved the problem by modifying the amount of ipv6 egress routes created (one per each NAT gateway). In this way I was able to deploy a single NAT Gateway for my 3 private subnets and associate to it a single ipv6 egress route.

To do so I've replaced local.len_private_subnets with local.nat_gateway_count
Here

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I just hit this issue, and this saved me hours since I probably wouldn't have noticed I kept "single_nat_gateway = true" in there by accident 😅, thanks @RunCor399 !

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Thank you @antonbabenko.

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I'm actually having the same problem, although in my setup I have 3 public, 3 private and 3 database subnets spread in 3 AZs

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I also just starting getting this error this week. I am testing possible workarounds. I suspect it is related to a recent Terraform code change.

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This issue has been resolved in version 5.7.1 🎉

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@andrewleegoss1978 It is all @bryantbiggs who reviewed it and merged it :)

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I'm going to lock this issue because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active issues. If you have found a problem that seems similar to this, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.

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