In this chapter, we will start to dive deeper into the management of network devices using Python. In particular, we will examine the different ways in which we can use Python to programmatically communicate with legacy network routers and switches.

What do I mean by legacy network routers and switches? While it is hard to imagine any networking device coming out today without an Application Program Interface (API) for programmatic communication, it is a known fact that many of the network devices deployed in previous years did not contain API interfaces. The intended method of management for those devices was through Command Line Interfaces (CLIs) using terminal programs, which were originally developed with a human engineer in mind. The management relied on the engineer's interpretation of the data returned from the device for appropriate action. As the number of network devices and the complexity of the network grew, it became increasingly difficult to manually manage them one by one. 

Python has two great libraries that can help with these tasks, Pexpect and Paramiko, as well as other libraries derived from them. This chapter will cover Pexpect first, then move on with examples from Paramiko. Once you understand the basics of Paramiko, it is easy to branch out to expanded libraries such as Netmiko. It is also worth mentioning that Ansible (covered in Chapters 7, The Python Automation Framework – Ansible Basics, and Chapter 8, The Python Automation Framework – Beyond Basics) relies heavily on Paramiko for its network modules. In this chapter, we will take a look at the following topics:

• The challenges of the CLI
• Constructing a virtual lab
• The Python Pexpect library

• The Python Paramiko library
• The downsides of Pexpect and Paramiko

Let's get started!