Automating Network Configuration with Python and Ansible

Network automation has evolved from a nice-to-have to an essential skill for modern network engineers. In this guide, I'll share practical approaches to automating network configuration using Python and Ansible.

Why Automate?

Manual network configuration is:

• Time-consuming and error-prone
• Difficult to scale across hundreds of devices
• Hard to maintain consistency
• Challenging to audit and rollback

Python for Network Automation

Python's extensive library ecosystem makes it ideal for network automation:

Using Netmiko for Device Configuration

python
from netmiko import ConnectHandler

device = {
    'device_type': 'cisco_ios',
    'host': '192.168.1.1',
    'username': 'admin',
    'password': 'secure_password',
}

with ConnectHandler(**device) as net_connect:
    output = net_connect.send_config_set([
        'interface GigabitEthernet0/1',
        'description Automated Configuration',
        'ip address 10.0.0.1 255.255.255.0',
        'no shutdown'
    ])
    print(output)

Parsing Configuration with TextFSM

TextFSM helps extract structured data from CLI output:

python
import textfsm

template = open('cisco_ios_show_interfaces.textfsm')
output = net_connect.send_command('show interfaces')

fsm = textfsm.TextFSM(template)
parsed_output = fsm.ParseText(output)

Ansible for Configuration Management

Ansible provides a declarative approach to network automation:

Basic Playbook Structure

yaml
---
- name: Configure Cisco Switches
  hosts: switches
  gather_facts: no
  
  tasks:
    - name: Configure VLANs
      cisco.ios.ios_vlans:
        config:
          - vlan_id: 10
            name: DATA
          - vlan_id: 20
            name: VOICE
        state: merged
    
    - name: Configure Interfaces
      cisco.ios.ios_interfaces:
        config:
          - name: GigabitEthernet0/1
            description: Uplink to Core
            enabled: true
        state: merged

Best Practices

1. Use Version Control

Store all automation scripts and playbooks in Git:

• Track changes over time
• Enable collaboration
• Facilitate rollback if needed

2. Implement Testing

Test configurations in a lab environment before production:

python
def test_vlan_configuration():
    output = device.send_command('show vlan brief')
    assert 'VLAN10' in output
    assert 'VLAN20' in output

3. Handle Errors Gracefully

python
try:
    net_connect.send_config_set(config_commands)
except NetmikoTimeoutException:
    logger.error(f"Timeout connecting to {device['host']}")
except NetmikoAuthenticationException:
    logger.error(f"Authentication failed for {device['host']}")

4. Use Jinja2 Templates

Create reusable configuration templates:

jinja2
interface {{ interface_name }}
 description {{ description }}
 switchport mode {{ mode }}
 {% if mode == 'access' %}
 switchport access vlan {{ vlan_id }}
 {% endif %}

Real-World Example: Bulk VLAN Deployment

Here's a complete example that deploys VLANs across multiple switches:

python
import yaml
from netmiko import ConnectHandler
from concurrent.futures import ThreadPoolExecutor

def configure_device(device_info):
    try:
        with ConnectHandler(**device_info) as net_connect:
            config = [
                'vlan 100',
                'name AUTOMATION_TEST',
                'exit'
            ]
            output = net_connect.send_config_set(config)
            return f"Success: {device_info['host']}"
    except Exception as e:
        return f"Failed: {device_info['host']} - {str(e)}"

# Load device inventory
with open('devices.yaml') as f:
    devices = yaml.safe_load(f)

# Parallel execution
with ThreadPoolExecutor(max_workers=10) as executor:
    results = executor.map(configure_device, devices)
    
for result in results:
    print(result)

Conclusion

Network automation with Python and Ansible transforms how we manage infrastructure. Start small with simple tasks like backup automation, then gradually expand to more complex workflows. The investment in learning these tools pays dividends in reduced errors, faster deployments, and better documentation.