Python Programming for Cisco Nexus (NX-OS) Series

Before I get into the Python for NX-OS, let me explain a few concepts that I've seen Network Engineers have been struggling with.

DevOps

DevOps is a software development method that combines the Software Development (Apps) and the IT Operations (Systems and Network).

GitHub

GitHub is the largest open source repository in the world. The typical operations that can be done from the Command line in Mac OS are shown below:

mjovanovic$ git

The most commonly used git commands are:

   add        Add file contents to the index

   bisect     Find by binary search the change that introduced a bug

   branch     List, create, or delete branches

   checkout   Checkout a branch or paths to the working tree

   clone      Clone a repository into a new directory

   commit     Record changes to the repository

   diff       Show changes between commits, commit and working tree, etc

   fetch      Download objects and refs from another repository

   grep       Print lines matching a pattern

   init       Create an empty Git repository or reinitialize an existing one

   log        Show commit logs

   merge      Join two or more development histories together

   mv         Move or rename a file, a directory, or a symlink

   pull       Fetch from and integrate with another repository or a local branch

   push       Update remote refs along with associated objects

   rebase     Forward-port local commits to the updated upstream head

   reset      Reset current HEAD to the specified state

   rm         Remove files from the working tree and from the index

   show       Show various types of objects

   status     Show the working tree status

   tag        Create, list, delete or verify a tag object signed with GPG

IDE (Integrated Development Environment)

IDE is like an advanced Text Editor. You may use Notepad++, but when you get deeper into Python – its highly recommended you switch to IDE. There are many, many versions out there, and the one I'm using and I recommend if you're-using Python is PyCharm (There is a community edition, which is free).

API (Application Programming Interface)

API is a set of requirements for the application interaction (how one application should use/interact with another app). The final aim here is to be able to do the Automation.

Again, what is an API? API, or an Application Programming Interface, is just a small webserver answering to xml/json requests. For programming the API interfaces you can use any language you want, and you will need to program/script because both leading SDN solutions on the market (Cisco ACI and VMware NSX) provide the programmability using the API interfaces.

Python can easily be used as a Scripting Language, which is why us, as Network Engineers, would choose it over other Languages. It´s an object oriented language like Java, and it relies heavily on the Exceptions.

So again, a million dollar question is - Why Python?

• Because it´s Readable (I don’t even want to remember all the code I wrote in PEARL while working for IBM)
• It´s easy to debug, cause every error has an Exception.
• It´s suitable for Scripting, which is what we need to automate some of our Networking procedurers, which are by nature repetitive.
• Procedural Programming is important to understand in order to get what Python is about. It is a set of steps (sub-routines) that are carried out in order to get a certain procedure done.
• Object Oriented Programming is even more important, because that’s what kind of language Python is. The building blocks are:
• Methods represent what the Object does.
• Functions
• Classes are like Blueprints. They only declare the attributes.
• Objects (an instance of a class).

There is also a Functional Programming, which is basically stateless; each Function operates independently from one ahother. Luckily, there wont be much about Functional programming here.

Software Development Lifecycle includes an entire methodology, but as Network Engineers, we need just a few core steps:

• Gather the Requirements, which is probably the most important part. This includes defining the Scope and you should be sticking exactly to the documentation created in this phase.
• Application Design, which later guides you how to write a code. It helps you break down the code. It´s very usefull to use the already created templates from your previous projects, or from the other designers.
• Implementation, which means - actually writing the code. Be sure to stick to requirements, and be sure to write the simple patterned code and comment/document your code. Make a personal database of the procedures you frequently use.
• Testing, and validating your specific parts of the code, and the entire code. When you write an API, which is what us, Network Engineers, will be doing most of the time when we're forced to program, the tests are mandatory.
• Version Control, if you accidentally delete a part of the code you allways need to have a backup.

REST API Model

REST is is an architecture style for designing networked applications. HTTP/S is used to make the CALLs, and the response will be the simple XML/JSON (popular data formats that are easy to read and parse) formatted information that we will later process.

Bash

TIP: On the VI editor, use “i” to get into Insert mode, and “Esc” to get back to the command mode. Use “/string” to search.

Bash stands for “Bourne-Again SHell”, and it’s the set of Linux commands on the Nexus Switch (End of 2015 – only supported on N9k). Bach enables access to the underlying Linux system on the device and to manage the system:

Nexus9k(config)# feature bash-shell

Nexus9k # run bash

bash-4.2$ whoami

admin

bash-4.2$ pwd

/bootflash/home/admin

TIP: You can use BASH to compile your Python Script, and protect your intellectual property, and prevent it from being modified, using the command “python –m compile FILE.py”. Don’t forget to remove the original (.py) file once you’ve created the compiled one (.pyc).

Bash Scripting can get more advanced too, so if you need to, for example, capture the receive counters of the interfaces:

for I in {1..5}

       do

               echo “RX Counters”

               date

       ifconfig eth1 | grep “RX packets” | cut –d ‘:’ –f2 | cut –d ‘ ‘ –f1 sleep 1

done

The biggest advantage when you compare Scripting with Python and the typical Switch CLI commands copy-paste that we all do is that when Python command fails, it stops and does a clean up.

When you start, you need to import the Cisco Module (library) to Python first:

>> import cli

or

>> from cli import *

*Cisco Module is available ONLY on the Nexus Switches, baked in. You cannot download it from the GitHub.

A procedure would go like this:

-        Write a Python script and copy it to a Bootflash

-        Option 1: Write a EEM, where the ACTION is the execution of the script you wrote

-                           “action 1 cli python bootflash:/script.py”

-        Option 2: Create a Scheduler, and execute the Python script in the desired moment (“feature scheduler” on the NX-OS).- You need to define a TASK and relate it to a Python script, and define the execution time:

(config)# scheduler job name TASK


(config-job)# python bootflash:/scripts/My_Script.py

(config)# exit


(config)# scheduler schedule name TASK


(config-schedule)# job name TASK


(config-schedule)# time start now repeat 0:0:1

The CLI Command on a Python API has many built-in scripts:

-        cli.cli: Passes CLI configurations, with a “;” as the delimiter between the different lines in the command script

-        cli.clid: returns JSON of the command output, which is more understandable by the Web Developers

-        cli.clip: Prints the command output to the Standard Output