Python for Network Engineers

Python Network Automation Scripts Examples

In this post I will cover in detail some Python for network engineers. These are scripts that I use on nearly a daily basis.

For each example that needs connectivity I will be using a basic topology of 20 x CSR routers.

I will also be using Netmiko to connect to the network devices. If you have not used Netmiko before you can check this quick run through of creating a connection to a Cisco router using Netmiko.

If you are being told you need to learn or implement network automation these scripts should get you started.

Using Python for network automation is becoming a must have skill in 2021. So you need to make sure you understand your objects, variables, methods, functions, dictionaries and lists. As one of the most popular programming languages in use today, there is so much information available, the scripts below should get you started.

If you are new to Python you can check out my Python Tutorial for Beginners

If you want to download all the scripts used below you can git clone the repository with this command:
git clone https://github.com/rogerperkin/python-scripts-for-network-engineers.git

Cisco Python Script Examples

All the script examples below are focussed on automating Cisco networking devices (mainly routers). The scripts can be used and changed as you require and will work on most devices, they are only used here on CSR routers as a lab example.

1. Cisco Python script to SSH to router

The first script I am going to show you is how to SSH to a router.

This script uses Netmiko to make the connection and then I will run ‘show ip interface brief’ command to verify the ip interfaces on the router.

Script on Github: ssh-to-router.py

from netmiko import ConnectHandler

#First create the device object using a dictionary
CSR = {
    'device_type': 'cisco_ios',
    'ip': '192.168.1.220',
    'username': 'roger',
    'password': 'cisco'
}

# Next establish the SSH connection
net_connect = ConnectHandler(**CSR)

# Then send the command and print the output
output = net_connect.send_command('show ip int brief')
print (output)

# Finally close the connection
net_connect.disconnect()

This is a very basic script, which first imports from Netmiko the ConnectHandler.

Note: To run this you must have first installed Netmiko – if you have not please watch the video at the top of this post.

We then define a device called CSR and provide the information that Netmiko requires, which are device_type, IP address and a username and password.

Then using the net_connect module you connect to the device CSR

Once connected we send the command ‘sh ip int brief’ to the router and save the value as output

Finally we print output and disconnect from the device.

2. SSH to multiple routers

Now we have a basic connection to a single router, it’s time to scale it out, we are now going to connect to 5 routers. But it could easily be 100 or 1000!

ssh-to-multiple-routers.py

For this I have created a new file called devices.txt which contains all the IP addresses of our 5 x CSR routers. We then use the same script but loop through the connection and pass in the IP of each router so we can get the ‘sh ip int brief’ output of all the routers.

192.168.1.220
192.168.1.221
192.168.1.222
192.168.1.223
192.168.1.224
192.168.1.225 

NOTE: do not press enter at the end of the last line. Otherwise your script will have problems at the end.

# SSH to Multiple Devices from devices file
from netmiko import ConnectHandler

with open('devices.txt') as routers:
    for IP in routers:
        Router = {
            'device_type': 'cisco_ios',
            'ip': IP,
            'username': 'roger',
            'password': 'cisco'
        }

        net_connect = ConnectHandler(**Router)

        print ('Connecting to ' + IP)
        print('-'*79)
        output = net_connect.send_command('sh ip int brief')
        print(output)
        print()
        print('-'*79)

# Finally close the connection
net_connect.disconnect()

If your are doing this on scale you would be using a Python Automation Framework like Nornir. This gives you the benefit of an inventory, much like Ansible.

However, it’s still good to understand how to perform this task manually before jumping into other topics.

3. Python Script to backup Cisco config

Now we have established how to connect to multiple devices and can run basic commands, the next step is to be able to save that output.

One of the first tasks people usually take on when they get started with network automation is to take a backup of a configuration file. This Python script does just that.

backup-router.py

from netmiko import ConnectHandler

#First create the device object using a dictionary
CSR = {
    'device_type': 'cisco_ios',
    'ip': '192.168.1.220',
    'username': 'roger',
    'password': 'cisco'
}

# Next establish the SSH connection
net_connect = ConnectHandler(**CSR)

#Discover the hostname from the prompt 

hostname = net_connect.send_command('show run | i host')
hostname.split(" ")
hostname,device = hostname.split(" ")
print ("Backing up " + device)

filename = '/home/roger/python-scripts-for-network-engineers/backups/' + device + '.txt'
# to save backup to same folder as script use below line and comment out above line 
# filename = device + '.txt'

showrun = net_connect.send_command('show run')
showvlan = net_connect.send_command('show vlan')
showver = net_connect.send_command('show ver')
log_file = open(filename, "a")   # in append mode
log_file.write(showrun)
log_file.write("\n")
log_file.write(showvlan)
log_file.write("\n")
log_file.write(showver)
log_file.write("\n")

# Finally close the connection
net_connect.disconnect()

Note: you need to change filename = to match your environment

Keep reading for more Cisco Python script examples.

4. Python script to backup multiple routers

Now we have a script that can backup a singe Cisco router it is very easy to take the script we used to connect to multiple routers and just add in the backup task to the for loop.

This script uses the devices.txt file again to loop through the list of IP’s and take a backup of each router.

All backups are placed in /backups but you can change the location of the backup by changing the path after filename.

backup-multiple-routers.py

# SSH to Multiple Devices from devices file
from netmiko import ConnectHandler

with open('devices.txt') as routers:
    for IP in routers:
        Router = {
            'device_type': 'cisco_ios',
            'ip': IP,
            'username': 'roger',
            'password': 'cisco'
        }

        net_connect = ConnectHandler(**Router)

        hostname = net_connect.send_command('show run | i host')
        hostname.split(" ")
        hostname,device = hostname.split(" ")
        print ("Backing up " + device)

        filename = '/home/roger/python-scripts-for-network-engineers/backups/' + device + '.txt'
        # to save backup to same folder as script use below line and comment out above line 
        # filename = device + '.txt' 

        showrun = net_connect.send_command('show run')
        showvlan = net_connect.send_command('show vlan')
        showver = net_connect.send_command('show ver')
        log_file = open(filename, "a")   # in append mode
        log_file.write(showrun)
        log_file.write("\n")
        log_file.write(showvlan)
        log_file.write("\n")
        log_file.write(showver)
        log_file.write("\n")

# Finally close the connection
net_connect.disconnect()

5. Config Generator using Jinja2.

Jinja2 is a templating language for Python developers. It is not just used for network automation, it can be used to create files from templates for any code.

The basic setup is you create a template file that contains placeholders where you define a value i.e. DNS-Server then you have another file that contains these values and when you run your python script it picks the values and inserts them into your template and outputs a completed configuration file which you can push to your network devices.

6. IP Valid Checker

This simple Python script uses the ipaddress module and will validate if an entered IPv4 or IPv6 address is valid or not.

import os, ipaddress 

os.system('cls')

while True:
    ip = input('Enter IP Address: ')
    try: 
        print(ipaddress.ip_address(ip))
        print('IP Valid')
    except: 
        print:('-' *50)
        print('IP is not valid')
    finally: 
        if ip =='q':
           print('Script Finished')
           break 

This page is under constant development and as I use a new script I will be adding it here.

If you have any ideas for more sample Python scripts for network engineers please drop me a comment below.

Topics to add:

netmiko, napalm, pyntc, telnet

Python Videos for Network Automation

Notes for development

Consider using pathlib when opening and writing to files
https://docs.python.org/3.9/library/pathlib.html

More Python Network Automation posts

• Ansible vs Python
• Should network engineers learn Python?
• Pyats Genie Tutorial
• Python Virtual Environment Tutorial
• Nornir Training

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