Attaching subnets to a routing table

When you view details of a routing table, you can also view details of attached subnets, or attach a subnet to apply routing table routes to a particular subnet. Note that a routing table can be associated with multiple subnets. However, each subnet can be assigned only one routing table. By default, any subnet that is not associated with a routing table is associated with the default routing table. You can also reassign the routing table of a subnet to another subnet.

You can attach a subnet to a routing table, or reassign a routing table to a particular subnet by using the console, CLI, API, or Terraform.

Attaching subnets to a routing table in the console

To attach a routing table to a subnet in the console, follow these steps:

From the IBM Cloud console, select the Navigation Menu , then click Infrastructure > Network > Routing tables. The Routing tables for VPC page appears.
Click the name of the routing table in which you want to view subnet details. Alternatively, you can click the number of attached subnets.
From the Subnets tab, click Attach subnet. The Attach subnets to routing table side panel appears.
Select a subnet from the drop-down list. The current routing table, IP range, and location of the subnet shows.
Click Attach.

To reassign a routing table to a subnet, follow these steps:

From the routing table details page, click the Subnets tab.
Click the Actions menu next to the subnet, then click Reassign routing table.
From the Reassign subnet routing table side panel, click the subnet that you want to assign to this routing table.
Click Reassign.

Attaching subnets to a routing table from the CLI

Before you begin, set up your CLI environment.

To attach subnets to a routing table from the CLI, run the following command:

ibmcloud is subnet-update SUBNET_ID --routing-table-id ROUTING_TABLE_ID

Where:

SUBNET_ID: Is the ID or name of the subnet you want to update.
ROUTING_TABLE_ID: Is the ID or name of the routing table that you want to assign the subnet to.

Attaching subnets to a routing table with the API

To attach subnets to a routing table with the API, follow these steps:

Set up your API environment.
Store the VpcId value in a variable to be used in the API command:
```
export VpcId=<your_vpc_id>
```

Attach a subnet to a routing table:

curl -X PUT "$vpc_api_endpoint/v1/subnets/$subnet_id/routing_table?version=$api_version&generation=2"
-H "Authorization: $iam_token"
-d '{
  "id": "37491549-51c0-4acc-9e7b-9ab3628e1a68"
    }'

Attaching subnets to a routing table with Terraform

To view details of all or a specific routing table with Terraform, follow these steps:

Set up your Terraform environment.

Use one of the following examples:

To attach a subnet to a routing table:

resource "ibm_is_subnet_routing_table_attachment" "example" {
  subnet        = ibm_is_subnet.example.id
  routing_table = ibm_is_vpc_routing_table.example.routing_table
}

For more information about the ibm_is_subnet_routing_table_attachment resource, see the Terraform Registry.

On an existing subnet resource, update the routing_table attribute that is specified by the identifier:

resource "ibm_is_subnet" "example" {
  name            = "example-subnet"
  vpc             = ibm_is_vpc.example.id
  zone            = "us-south-1"
  ipv4_cidr_block = "10.240.0.0/24"
  routing_table   = ibm_is_vpc_routing_table.example.routing_table
}

For more information about the routing_table resource, see the Terraform Registry.