The Nornir inventory system is built on three layers that make up its hierarchical structure. Each layer contributes to how devices are defined, grouped, and managed in an automated environment.

• Hosts:
  These represent individual network devices (e.g., routers, switches, firewalls). Every host entry includes attributes like IP address, platform, credentials, and any custom metadata required for automation tasks.
• Groups:
  Groups allow you to categorize multiple hosts with shared characteristics (like OS type, location, or vendor). Variables defined here are inherited by member hosts and may be overridden at the host level.
• Defaults:
  This is a global configuration layer where fallback variables are defined for all hosts. If a host or group doesn’t specify a variable, Nornir pulls it from this layer.

The inventory is typically stored using the SimpleInventory plugin in YAML format and structured like this:

inventory/
  hosts.yaml
  groups.yaml
  defaults.yaml

Each file serves a different layer of the inventory. Here’s a breakdown:

• hosts.yaml: Contains the actual devices to manage, along with their credentials and device-specific data.
• groups.yaml: Defines collections of devices that share common characteristics such as vendors or locations.
• defaults.yaml: Stores reusable default values like usernames, passwords, or global platform definitions.

This structure provides a scalable, modular way to organize network devices for automation — whether you are managing ten devices or thousands across multiple data centers.

Below is an example of how to build a SimpleInventory directory using YAML. This is a common inventory format for small to medium-sized deployments and is perfect for getting started with Nornir.

Step 1: Create the Inventory Directory Structure

inventory/
  ├── hosts.yaml
  ├── groups.yaml
  └── defaults.yaml

This structure separates host-specific settings, group variables, and global defaults for better readability and reusability.

Step 2: Define Devices in hosts.yaml

---
core_router1:
  hostname: 192.168.1.1
  port: 22
  username: admin
  password: secretpass
  platform: ios
  groups:
    - cisco_routers

core_switch1:
  hostname: 192.168.2.1
  port: 22
  username: admin
  password: secretpass
  platform: eos
  groups:
    - arista_switches

This file defines individual devices with attributes like IP, port, credentials, and group membership.

Step 3: Create Common Properties in groups.yaml

---
cisco_routers:
  platform: ios
  data:
    location: datacenter-west

arista_switches:
  platform: eos
  data:
    location: datacenter-east

Group files allow shared variables across multiple hosts. These can include physical location, vendor, or configuration templates.

Step 4: Set Global Defaults in defaults.yaml

---
username: defaultuser
password: defaultpass

Defaults are automatically applied to all hosts unless explicitly overridden at the host or group level.

This modular structure enhances scalability, reduces redundancy, and simplifies updates to shared variables across the environment.

Nornir offers a wide selection of inventory plugins, allowing you to source device data from simple files or dynamic systems like NetBox, CSV, or Ansible. Here’s a step-by-step example of how to select and configure inventory plugins for different use cases.

Step 1: Choose the Inventory Plugin for Your Needs

• SimpleInventory:
  Uses YAML files (hosts.yaml, groups.yaml, defaults.yaml). Ideal for straightforward, file-based setups.
• YAMLInventory:
  Lets you manually specify YAML paths, offering more flexibility in file structure.
• AnsibleInventory:
  Enables reusing existing Ansible inventory files, useful for teams migrating from Ansible.
• NetBoxInventory2:
  Dynamically loads inventory data from a NetBox API, perfect for environments tracked with NetBox.
• CSVInventory / ExcelInventory:
  Loads devices from CSV or Excel files for easy, tabular management at scale.
• NautobotInventory:
  Integrates inventory from Nautobot’s automation platform for organizations using it as their source of truth.

Step 2: Configure Inventory Plugin in config.yaml

Below are configuration examples for several popular plugins:

# Example: SimpleInventory (YAML files)
inventory:
  plugin: SimpleInventory
  options:
    host_file: 'inventory/hosts.yaml'
    group_file: 'inventory/groups.yaml'
    defaults_file: 'inventory/defaults.yaml'

# Example: NetBoxInventory2 (Dynamic NetBox inventory)
inventory:
  plugin: NetBoxInventory2
  options:
    nb_url: https://netbox.local:8000
    nb_token: "abcdefg12345"

# Example: AnsibleInventory (Reuse Ansible inventory files)
inventory:
  plugin: AnsibleInventory
  options:
    hostsfile: "inventory.yaml"

# Example: CSVInventory (Tabular CSV format)
inventory:
  plugin: CSVInventory
  options:
    csv_file: "inventory/devices.csv"

Each plugin may have its own options. Refer to the plugin’s documentation for all supported parameters.

Step 3: Initialize Nornir with the Configured Plugin

from nornir import InitNornir
nr = InitNornir(config_file="config.yaml")

With the chosen inventory plugin configured, Nornir will automatically load your devices and groups from the designated source, supporting both small static files and large dynamic datasets.

To maximize the efficiency, security, and scalability of your network automation with Nornir Inventory, follow these key recommendations:

Step 1: Maintain Complete Asset Visibility

• Ensure your inventory provides a comprehensive view of all network devices, including their locations, roles, configurations, and relationships. This visibility supports accurate targeting and expedites troubleshooting.

Step 2: Utilize Structured Grouping for Flexibility

• Organize devices into groups based on attributes such as platform, location, or function. Leverage group-level variables to reduce redundancy and simplify bulk operations.

Step 3: Emphasize Consistency and Standardization

• Use defaults and group variables to apply uniform configurations across devices. This approach reduces human error and ensures network-wide compliance.

Step 4: Leverage Dynamic Inventory and Integrations

• For rapidly changing or large environments, integrate Nornir with dynamic inventory sources (e.g., NetBox, IPAM, databases). This keeps your device inventory accurate and up to date with minimal manual intervention.

Step 5: Ensure Efficient Task Execution

• Base automation scripts on precise, up-to-date inventory data to ensure tasks are executed on the correct devices, reducing risk and increasing reliability.

Step 6: Prioritize Security and Risk Mitigation

• Regularly audit your inventory for outdated devices or configurations that may pose security risks, and automate remediation actions where possible.

Step 7: Plan for Adaptability and Growth

• Design your inventory structure to accommodate network changes, such as adding or relocating devices. Use automation workflows that can dynamically adapt to inventory updates.

By implementing these strategies, you ensure that your Nornir inventory remains a powerful foundation for all network automation efforts—driving efficiency, security, and operational consistency across your infrastructure.