Task 06 - Variables

Estimated Time to Complete: ~15 minutes

In this task you'll use variables in your Network as Code configurations. Variables let you define a value once (at the global, group, or device level) and reference it everywhere it's needed, so you can keep shared settings in one place while letting each device override only what's unique to it.

What you'll learn

By the end of this task you will have:

• Used ${VARIABLE_NAME} placeholders inside a global configuration
• Defined a global variable (${SITE}) that every device inherits
• Defined a per-device variable (${HOSTNAME}) that resolves differently on each device

Variables in Network as Code

Variables in Network as Code work similarly to variables in programming languages. You define a variable with a value at one level (device, device group, or global), and then reference it elsewhere using the ${VARIABLE_NAME} syntax. When the Network as Code module processes your configuration, it substitutes the variable references with their actual values.

Benefits of using variables:

• Consistency: Define a value once, use it everywhere
• Maintainability: Change a value in one place, it updates everywhere it's used
• Reusability: Same configuration template can be used with different variable values
• Device-specific customization: Each device can have its own variable values while sharing configuration structure

Use case: dynamic hostname in banner and system hostname config

In this example, you'll touch two different pieces of configuration on each device: the hostname and the login banner.

• You'll define the HOSTNAME variable per device.

• You'll use that variable to set the system hostname in the global configuration.

• You'll update the login banners on each device to display its own hostname. Instead of creating separate banner configurations for each device, you'll use the same global banner configuration with the HOSTNAME variable.

• Let Network as Code substitute the variable automatically in both the system hostname and banner configuration

Step 1: Update the global configuration file

First, update the global configuration to use a variable in the banner. Open data/global.nac.yaml in VS Code and update it with the following content:

---
iosxe:
  global:
    variables:
      SITE: "LTRXAR-2008 Lab"
    configuration:
      banner:
        login: |-
          #########################################
          #                                       #
          #   Welcome to the IOS XE as Code Lab!  #
          #                                       #
          #########################################
          Site:   ${SITE}
          Device: ${HOSTNAME}
      system:
        hostname: ${HOSTNAME}

Two kinds of variable reference at work here:

• ${SITE} - defined globally (at the iosxe.global.variables level). Every device inherits the same value unless a device explicitly overrides it. This is how you keep organization-wide constants (site name, location, lab ID, DNS domain) in exactly one place.
• ${HOSTNAME} - referenced globally, but you'll define it per-device in Step 2 so each device resolves it to its own value.

Banner config:

• banner: login: - the login banner shown on SSH connect.
• |- - YAML block-scalar indicator; preserves newlines and indentation inside the string while removing the trailing newline.

System config:

• system: hostname: - sets the IOS XE hostname CLI to the resolved value of ${HOSTNAME}.

The image below illustrates the updated global configuration in VS Code:

Step 2: Define variables at device level

Now you need to define the HOSTNAME variable for each device. Open data/devices/core.nac.yaml in VS Code and update it with the following content:

---
iosxe:
  devices:
    - name: core
      host: 198.18.130.10
      variables:
        HOSTNAME: core
      configuration:
        interfaces:
          loopbacks:
            - id: 0
              description: "Router-ID loopback"
              ipv4:
                address: 198.51.100.10
                address_mask: 255.255.255.255

What's new:

• variables: - section where you define device-specific variables.
• HOSTNAME: core - sets ${HOSTNAME} to core for this device only. The system.hostname and banner.login in global.nac.yaml will resolve to this value when the merged model is rendered for core.

The image below illustrates the device configuration with variables in VS Code:

Step 3: Add variables to other devices

Now add the HOSTNAME variable to the other device configuration files.

Update data/devices/border.nac.yaml:

---
iosxe:
  devices:
    - name: border
      host: 198.18.130.20
      variables:
        HOSTNAME: border

Update data/devices/access01.nac.yaml:

---
iosxe:
  devices:
    - name: access01
      host: 198.18.130.11
      variables:
        HOSTNAME: access01

Update data/devices/access02.nac.yaml:

---
iosxe:
  devices:
    - name: access02
      host: 198.18.130.12
      variables:
        HOSTNAME: access02

How variable substitution works

When Terraform processes your configuration, it performs variable substitution at each level, for each device.

iosxe:
  devices:
    - name: access01
      host: 198.18.130.11
      configuration:
        banner:
          login: |
            ######################################
            #                                    #
            #   Welcome to Network as Code Lab!  #
            #                                    #
            ######################################
            Device: access01               # ← ${HOSTNAME} resolved
        system:
          hostname: access01               # ← ${HOSTNAME} resolved

Variable precedence

Variables can be defined at multiple levels. When the same variable is defined at different levels, the more specific level takes precedence:

1. Device variables (highest precedence) - Override everything
2. Device group variables (medium precedence) - Override global
3. Global variables (lowest precedence) - Default values

This allows you to define default values globally and override them per device or device group when needed.

---
iosxe:
  global:
    variables:
      TIMEZONE: UTC  # Global default
  devices:
    - name: example-device
      variables:
        TIMEZONE: America/New_York  # Overrides global default

Step 4: Apply configuration

Open your WSL Ubuntu terminal and navigate to your project directory:

cd ~/nac-iosxe

Optionally, preview the changes Terraform will make:

terraform plan

In the plan output, look for four separate hostname changes - one per device - even though you only edited one banner template in global.nac.yaml. That's variable substitution producing four distinct renders from one source, which is the whole point of this task.

Apply the configuration:

terraform apply

Step 5: Verify variable substitution

After Terraform completes successfully, verify the configuration was applied correctly on each device.

Use Solar-PuTTY to connect and verify:

1. Open Solar-PuTTY from your desktop
2. Connect to the core switch
3. Verify the updated hostname and banner
4. Repeat for other switches (border, access01, access02) to see their specific hostnames

Each device shows its own hostname in the banner, demonstrating that the same template produced device-specific results.

Common variable use cases

Variables are powerful for many scenarios beyond hostnames (device identity). Here are some common use cases:

Environment variables

Environment variables can also be used in Network as Code configurations. They are defined outside of the configuration files and can be referenced using the syntax below:

---
iosxe:
  devices:
    - name: example-device
      system:
        enable_secret: !env ENABLE_SECRET
        enable_secret_type: "0"

In the example above, the ENABLE_SECRET environment variable is referenced and used for the device's enable secret.

What you've accomplished

In this task, you have:

• ✅ Learned how variables work in Network as Code configurations
• ✅ Updated the global configuration to use a variable placeholder
• ✅ Defined device-specific variable values
• ✅ Understood variable substitution and precedence rules
• ✅ Applied and verified variable-based configuration

What's next

Tasks 07-09 cover the three template types (model, file, cli) and are optional - they demonstrate advanced patterns but aren't prerequisites for any later task. If you're short on time, jump straight to Task 10 - Schema validation. Otherwise, continue with Task 07.

← Previous: Task 05 - Single device config · Next: Task 07 - Templates 'model'