Create a global subnet for external load balancing

This page guides you through how to create a global subnet in Google Distributed Cloud (GDC) air-gapped and use that subnet for an external load balancer (ELB).

Global subnets let you configure subnets that can use external load balancing operations across multiple zones in your GDC organization. Load balancing brings the benefits of improved performance, reliability, and availability of applications and services by distributing network traffic across multiple servers. For more information about global subnets for load balancing, see About subnets for load balancing.

This page is for developers within the application operator group who are looking to manage load balancing for their organization. For more information, see Audiences for GDC air-gapped documentation.

Before you begin

To create a global subnet and configure it for ELBs, you must have the following:

Own the project you are configuring the load balancer for. For more information, see Create a project.
The necessary identity and access roles:
- Ask your Organization IAM Admin to grant you the Load Balancer Admin (load-balancer-admin) role.
- Ask your Organization IAM Admin to grant you the Global Load Balancer Admin (global-load-balancer-admin) role.
- Ask your Organization IAM Admin to grant you the Subnet Organization Admin (subnet-org-admin) role.
- Ask your Organization IAM Admin to grant you the Subnet Project Admin (subnet-project-admin) role.
For more information, see Predefined role descriptions.

Create a parent global subnet

The parent global subnet you create in this section serves as the IP address pool from which your ELB IP addresses are sourced from. The subnet must include the ipam.gdc.goog/network-segment: data label. You can specify the subnet parent using the spec.parentReference.name field. You have two options for configuring the CIDR for this parent subnet:

Create a subnet using a static CIDR configuration
Create a subnet using a dynamic CIDR configuration

For more information on the difference between static and dynamic CIDR configurations, see Static and dynamic CIDR configuration.

Create a subnet using a static CIDR configuration

Use a static CIDR configuration when you require precise control over your IP address space. This subnet has a type of Branch. For more information on the root, branch, and leaf subnet types, see Subnet hierarchy.

To create a global parent subnet with a static CIDR configuration, add your chosen CIDR block to the spec.ipv4Request.cidr field:

kubectl --kubeconfig GLOBAL_API_SERVER apply -f - <<EOF
apiVersion: ipam.global.gdc.goog/v1
kind: Subnet
metadata:
  labels:
    ipam.gdc.goog/network-segment: data
  name: ELB_PARENT_SUBNET_NAME
  namespace: platform
spec:
  ipv4Request:
    cidr: STATIC_CIDR
  parentReference:
    name: PARENT_NAME
    namespace: platform
  propagationStrategy: None
  type: Branch
EOF

Replace the following:

GLOBAL_API_SERVER: the global management API server's kubeconfig path. For more information, see Global and zonal API servers. If you have not yet generated a kubeconfig file for the API server, see Sign in for details.
ELB_PARENT_SUBNET_NAME: the chosen name for your global parent subnet for the ELB.
STATIC_CIDR: the specific CIDR block you want to allocate for this parent subnet, such as 10.0.10.0/27.
PARENT_NAME: the name of the existing parent subnet from which this new subnet is created.

To configure this subnet to work with ELBs, you must create a leaf subnet for the ELB.

Create a subnet using a dynamic CIDR configuration

The dynamic CIDR configuration automatically allocates an available CIDR block of a specified size from the parent subnet. This simplifies IP address management, especially in larger environments. This subnet has a type of Branch. For more information on the root, branch, and leaf subnet types, see Subnet hierarchy. To create a global parent subnet with a dynamic CIDR, configure the spec.ipv4Request.prefixLength field with the chosen prefix length:

kubectl --kubeconfig GLOBAL_API_SERVER apply -f - <<EOF
piVersion: ipam.global.gdc.goog/v1
kind: Subnet
metadata:
  labels:
  ipam.gdc.goog/network-segment: data
  name: ELB_PARENT_SUBNET_NAME
  namespace: platform
spec:
  ipv4Request:
    prefixLength: PREFIX_LENGTH
  parentReference:
    name: PARENT_NAME
    namespace: platform
  propagationStrategy: None
  type: Branch
EOF

Replace the following:

ELB_PARENT_SUBNET_NAME: your chosen name for the ELB parent subnet, such as lb-global-lancer-elb-subnet.
STATIC_CIDR: the specific CIDR block you want to use, such as 10.0.10.0/27. This variable is only applicable for static CIDR configuration.
PARENT_NAME: the name of the existing parent subnet from which this new subnet is created, such as default-vpc-workload-cidr.
PREFIX_LENGTH: the chosen prefix length for the dynamically allocated CIDR such as 27. This variable is only applicable for the dynamic CIDR configuration.

To configure this subnet to work with ELBs, you must create a leaf subnet for the ELB.

Create a leaf subnet for the ELB

After setting up the global parent subnet, you must create a leaf subnet to allocate a single IP address for the global ELB service. This leaf subnet must have a type field value of Leaf and must reside in the same project namespace as your load balancer resources, such as the ForwardingRule, BackendService, and Backend.

To create the leaf subnet and link it to the ELB, follow these steps:

Create a leaf subnet with a prefixLength value of 32, as it's intended to allocate a single IP address. The parentReference value references the previously created parent global subnet:
```
kubectl --kubeconfig GLOBAL_API_SERVER apply -f - <<EOF
apiVersion: ipam.global.gdc.goog/v1
kind: Subnet
metadata:
  labels:
    ipam.gdc.goog/allocation-preference: default
    ipam.gdc.goog/network-segment: data
  name: ELB_IP_SUBNET_NAME
  namespace: PROJECT_NAMESPACE
spec:
  ipv4Request:
    prefixLength: 32
  parentReference:
    name: PARENT_REF
    namespace: platform
  type: Leaf
EOF
```
Replace the following:
- ELB_IP_SUBNET_NAME: your chosen name for the leaf subnet, such as lb-project-elb-ip.
- PROJECT_NAMESPACE: the Kubernetes namespace corresponding to your project where your ELB objects are located, for example, lb-project.
- PARENT_REF: the name of the parent subnet that this leaf subnet sources its IP address from, such as the parent global subnet you previously created.
Connect the newly created leaf subnet, which holds the allocated IP address, with the ForwardingRuleExternal resource of your ELB. In your ForwardingRuleExternal resource, update the spec.cidrRef.name field to reference the name of the leaf subnet you created in the previous step:
```
kubectl --kubeconfig GLOBAL_API_SERVER apply -f - <<EOF
apiVersion: networking.global.gdc.goog/v1
kind: ForwardingRuleExternal
metadata:
  name: FRI_NAME
  namespace: PROJECT_NAMESPACE
spec:
  ports:
    - port: PORT
      protocol: PROTOCOL
  backendServiceRef:
    name: BES_NAME
  cidrRef:
    name: LEAF_SUBNET_NAME
EOF
```
Replace the following:
- FRI_NAME: your chosen name for the ForwardingRuleExternalobject, such as nginx-elb-static-fr.
- PORT: the port number on which your ELB listens to for incoming traffic such as 80.
- PROTOCOL: the network protocol your ELB uses, such as TCP or UDP.
- BES_NAME: the name of the BackendService associated with this ForwardingRuleExternal resource such as nginx-bes.
- LEAF_SUBNET_NAME: the name of the leaf subnet you created in the previous step, such as lb-project-elb-ip.

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