Data Formats and Templates (JSON, YAML, Jinja2)

Network automation depends on separating data (what you want to configure) from logic (how to apply it). Data formats like JSON and YAML store device variables, while template engines like Jinja2 generate device-specific configurations from those variables.

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Why Separate Data from Templates?

Approach	Problem
Hardcoded configs	Every device needs a separate script; changes require editing code
Data + Template	Write the template once, feed in different data for each device

Data (YAML)          +    Template (Jinja2)     =    Device Config
─────────────             ──────────────              ──────────────
hostname: R1              hostname {{ hostname }}      hostname R1
mgmt_ip: 10.0.0.1        interface Mgmt0              interface Mgmt0
                           ip address {{ mgmt_ip }}     ip address 10.0.0.1

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JSON (JavaScript Object Notation)

JSON is a lightweight, text-based data format widely used by APIs and automation tools:

Syntax Rules

Rule	Example
Data is in key-value pairs	"hostname": "R1"
Strings use double quotes	"name": "switch01"
Numbers have no quotes	"vlan_id": 100
Booleans are lowercase	"enabled": true
Arrays use square brackets	["Gi0/0", "Gi0/1"]
Objects use curly braces	{"hostname": "R1"}
No trailing commas	Last item has no comma
No comments allowed	Use a separate field if needed

Example: Device Inventory

{
  "devices": [
    {
      "hostname": "core-rtr-01",
      "ip": "10.0.0.1",
      "platform": "cisco_ios",
      "site": "London",
      "interfaces": [
        {
          "name": "GigabitEthernet0/0",
          "ip": "10.0.0.1",
          "mask": "255.255.255.252",
          "description": "Uplink to ISP"
        },
        {
          "name": "GigabitEthernet0/1",
          "ip": "10.0.1.1",
          "mask": "255.255.255.0",
          "description": "LAN segment"
        }
      ]
    }
  ]
}

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YAML (YAML Ain't Markup Language)

YAML is a human-friendly data format favoured by automation tools like Ansible:

Syntax Rules

Rule	Example
Key-value pairs use colon-space	hostname: R1
Indentation defines structure (spaces, not tabs)	Two-space indent is standard
Lists use a dash prefix	- GigabitEthernet0/0
Strings usually need no quotes	hostname: switch01
Comments use #	# This is a comment
Booleans: true / false	enabled: true
Multi-line strings use `	(literal) or>` (folded)

Example: Same Inventory in YAML

devices:
  - hostname: core-rtr-01
    ip: 10.0.0.1
    platform: cisco_ios
    site: London
    interfaces:
      - name: GigabitEthernet0/0
        ip: 10.0.0.1
        mask: 255.255.255.252
        description: Uplink to ISP
      - name: GigabitEthernet0/1
        ip: 10.0.1.1
        mask: 255.255.255.0
        description: LAN segment

JSON vs YAML Comparison

Feature	JSON	YAML
Readability	Moderate (brackets everywhere)	High (clean, minimal syntax)
Comments	Not supported	Supported (#)
Used by	APIs, JavaScript, config files	Ansible, Kubernetes, CI/CD
Strictness	Strict (quotes required for strings)	Flexible (quotes often optional)
Parsing speed	Fast	Slightly slower
Human editing	Harder with large files	Easier

Tip: Use YAML for files that humans edit (inventories, playbooks). Use JSON for API responses and machine-generated data.

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Jinja2 Templates

Jinja2 is a powerful template engine for Python. In network automation, it generates device configurations from templates and variables:

Installation