EC2. Load balancer. At least two subnets must be specified

Question

I'm trying to create and configure Load Balancer. The Availability Zones section has only one subnet and one zone for me.

I faced error:

At least two subnets must be specified.

Simple question - what should I do? thanks.

Sounds like you need to create at least one more subnet in a different AZ for your region. Once created, it should be selectable when configuring your ELB. — dmulter, Sep 13 '18 at 20:43
@dmulter thanks, that worked. Key point was a different AZ for region. — , Jun 08 '21 at 21:08

score 17 · Accepted Answer · answered Sep 13 '18 at 21:01

17

You'll first need to create two or more subnets in your VPC. It's typically good practice to create at least one subnet for each availability zone (AZ) in your region. Be sure to select public/private as is appropriate for your architecture.

There isn't a specific example in the AWS documentation, but be sure to first understand VPC concepts and the creation of subnets. See Scenarios and Examples for the closest example walkthroughs for creating subnets.

Selecting subnets when configuring an ELB should be straightforward now.

answered Sep 13 '18 at 21:01

dmulter

2,608
3
15
24

Can I use the same subnets for two ELBs when each of them is pointing to different instance? – BartusZak Jul 11 '19 at 07:29
@BartusZak - yes you can. – kiran01bm Jun 07 '20 at 13:00

score 4 · Answer 2 · answered Sep 11 '19 at 08:13

You need two subnets in your VPC: good practice is to have one in two different availability zones, for instance a and b. The relevant section of TERRAFORM_CONFIG_NAME.tf would look something like this:

setting {
    namespace = "aws:ec2:vpc"
    name      = "Subnets"
    value     = "${lookup(var.vpc_subnets, format("%s_%s", var.location, var.availability_zone))}, ${lookup(var.vpc_subnets, format("%s_%s", var.location, var.secondary_availability_zone))}"
}

, variables.tf like this:

variable "vpc_subnets" {
  type = "map"
  default = {
    "frankfurt_a" = "subnet-12345671"
    "frankfurt_b" = "subnet-12345672"
    "frankfurt_c" = "subnet-12345673"
    "ireland_a" = "subnet-12345674"
    "ireland_b" = "subnet-12345675"
    "ireland_c" = "subnet-12345676"
  }
}

variable "availability_zone" {
  default = "a"
}

variable "secondary_availability_zone" {
  default = "b"
}


variable "regions" {
  type = "map"
  default = {
    "frankfurt" = "eu-central-1"
    "ireland" = "eu-west-1"
    "london" = "eu-west-2"
  }
}

variable "location" {
  default = "ireland"
}

Alternatively, it is also possible to simply provide a list of subnets in the value field of the TERRAFORM_CONFIG_NAME.tf like: `value = "subnet-1, subnet-2, subnet-3"`. — Jonas, Sep 11 '19 at 10:34

score 1 · Answer 3 · answered Mar 13 '23 at 23:54

1

There is an easier way to solve this problem. See the docs code below as an example.

resource "aws_lb" "test" {
  name               = "test-lb-tf"
  internal           = false
  load_balancer_type = "application"
  security_groups    = [aws_security_group.lb_sg.id]
  subnets            = [for subnet in aws_subnet.public : subnet.id]

  enable_deletion_protection = true

  access_logs {
    bucket  = aws_s3_bucket.lb_logs.id
    prefix  = "test-lb"
    enabled = true
  }

  tags = {
    Environment = "production"
  }
}

Hopefully this will help

answered Mar 13 '23 at 23:54

augusto valdivia

11
3

Can you please clarify what makes it "easier" in the code provided? – AlexHalkin Mar 20 '23 at 15:34
1

In the past, you needed to use for_each perhaps I meant you can still do it. however one can only use one line: `subnets = [for subnet in aws_subnet.public : subnet.id]` To avoid the complexity. Hopefully this answered your question – augusto valdivia Mar 20 '23 at 21:10

EC2. Load balancer. At least two subnets must be specified

3 Answers3