Notes on the Oreilly book Network Programmability and Automation

What is Network Automation?

  • Simplifying tasks involved in configuring, managing and operating network equipment, network topologies, network services and network connectivity.
  • Continuous Integration
  • Configuration management
  • Source code control

Benefits of Reading this book

  • Network Engineers - fluent in network protocols, configuring network devices, operating and managing a network
  • System Administrators - responsible for managing systems that conenct to the network
  • Software Developers - Useful to see developer tools and lanugages in a networking context

1. Network Industry Trends

The Rise of Software Defined Netoworks

Martin Casado - influencing large network incumbents that operations, agility and managability must change


  • First major protocol for software defined networks
  • Decoupled network device’s control plane (brain’s of device) from the data plane (hardware doing the packet forwarding)
  • Hybrid mode - deployed on a given port, VLAN or a normal paket forwarding pipeline
  • Low level and directly interfaces with hardware tables instructing how a network device should forward traffic
  • Not intended to interact with the management plane - like authentication or SNMP
  • Policy Based routing - forward traffic based on non-traditional attributes, like a packet’s source address.
  • Achieve same ganularity of packet forwarding but in a vendor-neutral way

Why Openflow?

  • Network devices were closed (locked from installing third-party software) and only had a CLI
  • CLI’s although well known and even preferred by administrators - it did not offer the flexibility to manage, operate and secure a network
  • The biggest change in the last 20 years is a move from telnet to SSH
  • Management of networks has lagged behind other technology comparitively for both configuration management and data gathering and analysis
    • hypervisor managers, wireless controllers, IP PBX’s, Powershell and DevOps Tools
  • Question’s openflow was based around:
    • Was it possible to redirect traffic based on the application
    • Did network devices have an API?
    • Was there a single point of communication to the network?

What is Software Defined Networking (SDN)?

  • SDN is similar to what cloud was a decade ago: IaaS, PaaS and SaaS.
  • The definition is kind of up in the air
  • Things in SDN:
    • Openflow
    • Network Functions Virtualisation
    • Virtual switching
    • Device API’s
    • Network Automation
    • Bare-metal switching
    • Data centre network fabrics
    • SD-WAN
    • Controller Networking


  • Vendor independence from controller software - a NOS (Network Operating System)
  • Vendors who use openflow (Big Switch Netowrks, HP, NEC) have developed extensions
  • The use of openflow or any other protocol is less important than the business being supported

Network Functions Virtualisation (NFV)

  • Deploying traditionally hardware functions as software
  • Most common example is virtual machines that operate routers, firewalls, load balancers, IDS/IPS, VPN, application firewalls etc.
  • Helps to issue of having to deploy expensive hardware to future proof and reducing complexity for future upgrades.
  • A software based NFV lets you pay as you grow and minimises the failure domain
  • For example deploy many Cisco ASAv applicances instead of a single large Cisco ASA.
  • Time needed to rack, stack, cable and integrate hardware is eliminated, it becomes as fast as deploying a virtual machine and an inherent advantage is being able to clone and backup environments.

Why hasn’t NFV taken over: * Requires a rethink on how the network is operated - for example a single big firewall as oppsed to many multi-tenant firewalls * Or a single CLI or GUI, making the failure domain immense but streamlines tha administration. * In modern network automation, it matters less what architecture is chosen as management of devices is becoming easier * Vendors are delibrately limiting the performance of their virtual application-based technology

Virtual Switching

Common virtual switches: * VMWare standard switch (VSS) * VMWare distributed switch (VDS) * Cisco Nexus 1000V * Cisco Application Virtual Switch (AVS) * Open vSwitch (OVS)

Software based switches that reside in the hypervisor kernel providing local network connectivity between virtual machines and containers.

Features: * MAC Learning * link aggregation * SPAN * sFlow

New access layer or edge within a datacentre. It is no longer the physical TOR (top-of-rack) switch with limited flexibility

Makes it easier to distribute policy throughout the network

Network Virtualisation

Software-only overlay-based solutions

solutions: * VMWare’s NSX * Nuage’s Virtual Service Platform (VSP) * Juniper’s Contrail

An overlay based solution like Virtual Extensible LAN (VxLAN) is used to build connectivity between hypervisor based virtual switches.

This connectivity and tunneling approach provides Layer 2 adjacency between virtual machines that exist on different physical hosts independent of the physical network, meaning the physical network could be Layer 2, Layer 3, or a combination of both.

The result is a virtual network that is decoupled from the physical network and that is meant to provide choice and agility.

  • more than just virtual switches being stitched together by overlays
  • comprehensive, offering security, load balancing, and integrations back into the physical network all with a single point of management
  • offer integrations with the best-of-breed Layer 4–7 services companies
  • No need to configure virtual switches manually, as each solution simplifies this process by providing a central GUI, CLI, and also an API where changes can be made programmatically

Device API’s

  • Vendors realised that using a CLI has severely held back operations
  • Main issue is scripting does not return structured data - it is returned in raw text that had to be parsed
  • If the output of show commands changed slightly the scripts would break
  • Strucuted data returned eliminates the need to parse the text giving a cleaner interface to develop and test code

Test code could mean testing new topologies, certifying new network features, validating particular network configurations, and more

The most popular API is by Arista Networks called eAPI - a JSON over HTTP Api. Cisco brought out the Nexus NX-API and NETCONF/RESTCONF

Nearly every vendor has some sort of API these days

Network Automation

Not just about automating the configuration of network devices but also acessing data in network devices: * Flow level data * routing tables * FIB tables * Interface statistics * MAC tables * VLAN tables * serial numbers

Time to debug and troubleshoot is reduced

Streamlines the process of every network admin having their own bet practices

Bare Metal Switching

This is not SDN (Software defined Networking)

Network devices were always bought as a physical device - as hardware appliances, an operating system and features you can use on the system - all from the same vendor.

With white-box or bare metal network devices the device looks more like a x86 server - allowing you to pick and choose the vendor you want to use.

Companies solely focused on white box switching - software: * Big Switch Networks * Cumulus Networks * Pica8

Whitebox hardware platforms: * Quanta * Super Micro * Accton

In a bare metal device components: application, operating system and hardware and disaggregated

If there is a controller integrated with the solution using a protocol such as OpenFlow and is programmatically communcating with network devices that gives it a Software Defined Networking flavour.

If there is no controller requirements then it makes it a non-SDN based architecture

In short whitebox and baremetal switching gives flexibility to change designs, architecture and software without swapping hardware just changing the operating system

Data Centre Network Fabrics

Changes the mindset of network operators from managing individual boxes one at a time to managing the system in its entirety. An upgrade is a migration from system to system, or fabric to fabric.

Examples: * Cisco’s Application Centric Infrastructure (ACI) * Big Switch’s Big Cloud Fabric (BCF) * Plexxi’s Fabric and Hyperconverged network

Attributes of data centre networking fabrics: * A single interface to manage or configure the fabric - including policy management * Distributed default gateways across the fabric * Multi-pathing capabilities * They use some form of SDN controller


Software Defined Wide Area Networking

Vendors: * Viptela * CloudGenix * VeloCloud * Cisco IWAN * Glue Networks * Silverpeak

Offers more choice

Controller Networking

A characteristic to deliver modern solutions

OpenDayLight - popular SDN controlller - it is a platform not a product. Can be used for network monitoring, visibility, tap aggregation etc. beyond fabrics, network virtualisatoin and SD-WAN

2. Network Automation

Why Network Automation?

  • Speed
  • Simplified Artchitectures
  • Deterministic Outcomes
  • Business Agility

Simplified Artictures

Most network devices are uniquely configured (as snowflakes) and network engineers take pride in solving transport and application issues with one off changes - is makes the network harder to maintain, manage and automate.

Network automation needs to be included from the outset of new architectures (not just an add-on)

Archiecture becomes simpler, repeatable, easier to maintain and automate.

Still necessary to eliminate one-off changes

Deterministic Outcomes

  • The impact of typing the wrong command can be catastrophic
  • Each engineer has their own way of making a particular change
  • Using proven and tested network automation makes changes more predicatable

Business Agility

Always understand existing manual workflows, document them and understand the impact they have to the business Then deploying automation chnology and tooling becomes much simpler

Types of Network Automation

Device Provisioning

  • Fastest way to get started is to automate the creatoin of device configuration files for initial device provisioning and push them to network devices
  • Decouple the inputs from the vendor-proprietary syntax
  • A seperate file for values of configuration parameters - a configuration template

Data Collection

  • Monitoring tools typically use the SNMP (Simple Network Management Protocol)
  • Newer devices use a push model which streams telemetry to a server of your choosing


The beautiful thing is that a migration tool such as this is much simpler to build on your own than have a vendor do it because the vendor needs to account for all features the device supports as compared to an individual organization that only needs a finite number of features. In reality, this is something vendors don’t care much about; they are concerned with their equipment, not making it easier for you, the network operator, to manage a multi-vendor environment.

Only you, not the large networking vendors, have the motivation to make multi-vendor automation a reality.

It is important to think about the tasks and document them in human readable format that is vendor neutral

Configuration Management

  • Deploying, pushing and managing configuration state of a device

The great power comes with great responsibility - tests must always be performed before rolling out to production environments


It is easier to start with data collection, monitoring and configuration building which are read only and low risk actions

A low risk use case is configuration compliance checks and configuration validations

  • Does it meet security requirements?
  • Are the required networks configured?
  • Is protocol XYZ disabled?

What happens when it fails compliance - is it logged, is anyone notified, does the system autocorrect? Event-driven automation

It is always best to start simple with automation


  • Custom and Dynamic Reports
  • Data being returned becomes input to other configuration management tasks
  • Reports can be produced in any format


  • Automated troubleshooting becmoing a reality
  • Troubleshooting interrupts learning and improving work

The trick is how troubleshooting is done: * Do you have a personal methodology? * Is the method consistent with al members of the team? * Does everyone check Layer 2 before troubleshooting Layer 3? * What steps are taken?

Eg. Troubleshooting OSPF (A routing protocol used to connect with other routers) * What does it take to form an OSPF adjecency between 2 devices? * Can you say the same answer at 2 in the morning? * Do you rmember that some devices need to be on the same subnet, have the same MTU and have consistent times and same OSPF network type?

Other examples: * Can particular log messages correlate to known conditions on the network? * BGP neightbour adjacencies, how is a neighbour formed? * Are you seeing all the routes you think should be in the routing table? * What about VPC and MLAG configurations? * What about port-channels? Are there any inconsistencies? * Do neighbours match the port-channel configuratoin (going down to the vSwitch)? * Cabling - are cables plugged in correctly?

Evolving the Management PLane from SNMP to Device API’s

API’s (Application Programming Interfaces)


  • A protocol used to poll network devices for information about status, CPU, memory and interface utilisation
  • There must be an SNMP agent on a managed device and a network managment station (NMS) - which acts as the server for managed devices.
  • This SNMP data is described and modelled in MIB (Management Information bases)
  • SNMP supports both Get Requests and Set Requests (PATCH/POST)
  • Not many vendors offer full support for the configuration management via SNMP - they often used custom MIB’s
  • Some vendors are claiming the gradual death of SNMP - although it does exist on nearly every network device
  • There are python libraries for SNMP

SSH/Telnet and the CLI

  • The CLI was built for humans - not meant for machine-to-machine comms
  • Raw text returned from a show command is not formatted or structured
  • SSH/CLI makes automation extremely error prone and tedious


  • A network management layer protocol - like SNMP to retrieve and change configuration
  • Leverages SSH
  • Data sent between a NETCONF client and NETCONF server is encoded in XML
  • RPC’s (Remote Procedure Calls) are encoded in the XML document using the <rpc> element
  • RPC’s map directly to NETCONF operations and capabilities on the device
  • Supports transaction based exchanges - if any single change fails everything is rolled back

Restful API’s

  • Representational State Transfer
  • Network controllers
  • The web server is the network device or SDN controller
  • You then send requests to that server with a client

Impact of Open Networking

All things open: * Open source * Open networking * Open API’s * OpenFlow * Open Computer * Open vSwitch * OpenDaylight * OpenConfig

  • It improves consistency and automation
  • Many devices support python on-box
  • Meaning you can go into the python interpreter and run python scripts locally on the device
  • More robust API’s are supported (Netconf and REST instead of SNMP and SSH)
  • Network devices are exposing more of the Linux Internals - use ifconfig, apt or yum

Network device API’s that exist now that didn;t a few years ago: * Cisco NX-API * Arista eAPI * Cisco IOS-XE * RESTCONF/NETCONF

Network Automatino in the SDN Era

  • Even with controllers network automation is imortant
  • Cisco, Juniper, VmWare, Big Switch, Plexxi, Nuage, Viptela offer controller platforms. Not to mention OpenDaylight and OpenContrail
  • Important to avoid making error-prone changues with the GUI

3. Linux

Linux in the Network Automation Context

  • Several network operating systems are based on Linux
  • Some are bringing full Linux distributions targeted at network equipment
  • Open Network Linux - Big Switch’s Switch Light is an example built on Open Network Linux
  • Many tools have origins in Linux
  • You will often use anisble from a computer using Linux

Brief History of Linux

  • 1980’s Richard Stallman launched the GNU Project to provide a free Unix-like operating system - GNU
  • A wide collection of Unix utilities and applications were created but the kernel “GNU Hurd” never gained momentum
  • Linus Torvald tried to create a MINIX clone in 1991 - the start of Linux
  • GNU/Linux is the OS utilities and the kernel

Linux Distributions

Red Hat, Defora and CentOS

  • Red Hat offered Red Hat Enterprise Linux (RHEL) along with technical support
  • The fast pace of Linux development is often at odds with slowed and more methodical pace required for stability and reliability by the Red Hat paltform
  • Fedora is the upstream distribution of RHEL - so Fedora has all the stuff new
  • To avoid the RHEL costs, an open source clone was made called CentOS (Community Enterprise Linux)
  • These distributions share a common package format - RPM

Many distributions replaced the rpm package manager with yum (Yellowdog updater) and are now moving to a tool called dnf (Dandified YUM)

Other distributions also use the RPM format: Oracle Linux, Scientific Linux and Suse derivatives

Debian, Ubuntu and others

  • Debian GNU/Linux is a distribution produced and maintained by the Debian project
  • Founded in 1993 by Ian Murdock
  • Three branchesL stable, testing and unstable
  • Ubuntu Linux started in 2004 - funded by canonical by Mark Shuttleworth
  • Has desktop, server and mobile focused versions
  • Stick to LTS (Long term support) releases for best practice
  • They use teh .deb package and use the dpkg tool
  • Recently apt, apt-get and aptitude are used

Interacting with Linux

  • Receive IP addresses via a Linux based DHCP server
  • Access a linux powered web server like apache
  • Utilise DNS to resolve domain names to IP addresses
  • The most common shell is bash - bourne again shell
  • A single-root filessytem - all drives and directories fall into a single namespace
  • Linux treats everything as a file - even storage devices, ports, IO
  • Every file has a unique path to its location

ping is found in /bin/ping

arp is found in /usr/sbin/arp

~ is a shortcut to a user’s home directory

The prompt: ubuntu@backup:~$

Denotes ubuntu user on the jessie hostname currently in the home directory. The $ at the end means that the currect user does not have root permissions

  • pwd - print working directory - prints the full path of the directory you are in
  • cd x - change directory. A leading slash indicated from teh root, otherwise it is relative to the current location. Use cd .. to move up one directory in the hierachy.
  • . - the currect directory

Top tip: cd - tells bash to switch back to the last directory you were.

Top tip: cd shortcuts to the home directory

The search path are places linux automatically searches when you type a command. Typical locations inlcude /bin, /usr/bin and /sbin. Being specific about which file to run using the absolute path ./ is important.

Manipulating Files and Directories

  • touch x - create files
  • mkdir x - make directory
  • rm x - remove a file
  • rm -R x - remove a directory
  • cp x y - copy file
  • cp -R x y - copy a directory

There is no recycle bin or trash can. Be vary careful with rm, cp and mv. Overwritten files are gone.

You can always get your own help using man pages: eg. man cp


  • permissions are assigned based on the user, group and others
  • permissions are based on action (read, write, execute)

Each action has a value: * 4 is read * 2 is write * 1 is execture

To allow for multiple actions add the underlying values

The values are assigned to user, group and others.

  • 644: user read and write, group read, others read
  • 755: user read, write and execute; group read and execute; others read and execute
  • 620: user read and write, group write, others none

A line like rxwr-xr-x breaks down read, write and execute into user, group and other

  • ls -l - view files and permissions in a directory
  • chmod - change permissions
  • chown - change ownership of a file


chmod 755 bin # sets bin directory to 755
chmod u+rw config.txt # adds read and write permission to the user that owns the file
chmod u+rw, g-w /opt/share/config.txt # adds read and write for the user, remove write for the group

Running Programs

What makes an executable file? It could be binary, compiled from C or C++ It could also be an executable text file, such as a bash shell script or a python script

The file utility can help to tell you what file of file it is

ubuntu@backup:~$ file backup.log 
backup.log: UTF-8 Unicode text
ubuntu@backup:~$ file Bourne-Again shell script, ASCII text executable

The shebang - the first line in a text-based script and starts with !followed by the path to the interpreter. Eg. !/usr/bin/python - tells bash which program to use

You can view your search path with:

echo $PATH

The search path is controlled by the environment variable: PATH

To find where to absolute path to an executable is:

which uptime

Working with Daemons

  • In the linux world the term daemon refers to a process that runs in the background. Also sometimes called a service
  • Daemons are most often encountered when dealing with network related services
  • Working with daemons used to vary depending on the distribution
  • Startup scripts callen init scripts were used to start, stop and restart a daemon.

On some systems the service utility is used - behind the scenes this utility is calling distributino specific command - initctl on ubuntu and systemctl on centOS

In recent years major linux distro’s have converged on the use of systemd as teh init system.

Prior to debian 8, it used System V init

Working with Daemons on Debian 8

  • Using the systemctl utility
  • Debian does not provide a user friendly wrapper

Start a service:

systemctl start service-name

Stop a service:

systemctl stop service-name


systemctl restart service-name

Reload (config) - less disruptive than restarting:

systemctl reload service-name

View status of daemon:

systemctl status service-name

View all the services:

systemctl list-units

On Ubuntu

Similar to the above except using initctl

Background services on CentOS 7.1

  • Same as debian core systemctl commands
  • centOS includes the service wrapper script

Show the network connections to a daemon, use ss. To show listening network sockets to ensure that network configuration is working properly

ss -lnt # tcp sockets
ss -lnu # udp sockets

Information about currently running proccesses:


Networking in Linux

Working with Interfaces

  • Physical interfaces
  • VLAN interfaces
  • Bridge interfaces

Configured using the CLI or config files

Interface configuration via command line

Most linux distributions have configured on a single set of command line utilities for working with network interfaces

Part of the iproute2 set of utilities (On centos it is known as iproute) These utilities use ip to replace the functionality of the deprecated ifconfig and route

For interface config 2 sub commands to the ip command will be used: * ip link - view or set interface link status * ip addr` - view or set ip addressing configuration on interfaces

Listing interfaces:

ip link list
ip addr
id link


stephen@web:~$ ip addr
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default 
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet scope host lo
    valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host 
    valid_lft forever preferred_lft forever
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 04:01:29:7e:44:01 brd ff:ff:ff:ff:ff:ff
    inet brd scope global eth0
    valid_lft forever preferred_lft forever
    inet6 fe80::601:29ff:fe7e:4401/64 scope link 
    valid_lft forever preferred_lft forever

Output shows: * the current list of interfaces * the current maximum transmission unit (MTU) * the current administrative state (UP) * ethernet media access control (MAC) address

The status in the angled brackets <> can be: * UP - Indicates the interface is enabled * LOWER_UP - Indiciates the interface link is up * NO_CARRIER - The interface is enabled but there is no link (THe interface is “down”) * DOWN - THe interface is administratively down

To filter for a specific interface:

ip link list interface


stephen@web:~$ ip link list eth0
2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP mode DEFAULT group default qlen 1000
link/ether 04:01:29:7e:44:01 brd ff:ff:ff:ff:ff:ff

Listing is the default. Ie. ip route will list all the routes

CentOS assigns different names to the interfaces than Debian and Ubuntu

Disabling an interface:

ip link set <interface> down

[vagrant@centos ~]$ ip link set ens33 down
[vagrant@centos ~]$ ip link list ens33
3: ens33: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT
    qlen 1000
    link/ether 00:0c:29:d7:28:21 brd ff:ff:ff:ff:ff:ff

The state DOWN and lack of NO_CARRIER, tells you the interface is administratively down and not just down used to a link failure

Enabling an interface

ip link set <interface> up


[vagrant@centos ~]$ ip link set ens33 down
[vagrant@centos ~]$ ip link list ens33
3: ens33: <BROADCAST,MULTICAST> mtu 1500 qdisc pfifo_fast state DOWN mode DEFAULT
qlen 1000
link/ether 00:0c:29:d7:28:21 brd ff:ff:ff:ff:ff:ff

Setting the MTU of an interface

ip link set mtu <number> <interface>


[vagrant@centos ~]$ ip link set mtu 9000 ens33

This change is immediate but not persistent

Assigning an IP address to an interface

ip addr add address dev interface


vagrant@jessie:~$ ip addr add dev eth1

If an interface already has an IP address assigned, the ip addr add command simply adds the new address, leaving the original address intact.

Remove an ip address:

ip addr del address dev interface

vagrant@jessie:~$ ip addr del dev eth1

We have been modifying the running configuration we haven’t made these configuration changes permanent. In other words, we haven’t changed the startup configuration. To do that, we’ll need to look at how Linux uses interface configuration files.

Interface configuration via configuration files

interface configuration files across different Linux distributions can be quite different

On RHEL, CentOS and Fedora configuration filess are found at: /etc/sysconfig/network-scripts

The configuration files are named: ifcfg-<interface>

An example may look like this:

  • NAME: A friendly name for users
  • DEVICE: Name of physical device being configured
  • IPADDR: Ip address to be assigned (if not using DHCP or BootP)
  • PREFIX: Network prefix for the assigned IP addres (an use NETMASK)
  • BOOTPROTO: How the ip address will be assigned, a value of dhcp can be used. none means statically defined.
  • ONBOOT: yes will activate the device at boot time. Setting no will not.
  • MTU: Specifies default MTU for the interface
  • GATEWAY: Specifies gateway to be used

For full detauls you can check: /usr/share/doc/initscripts-<version>/sysconfig.txt on a CentOS system.

On debian and derivatives, interface configuration is handled at /etc/network/interfaces

We can use cat to show the contents on the screen:

cat /etc/network/interfaces


stephen@web:~$ cat /etc/network/interfaces
# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

# The primary network interface
auto eth0
iface eth0 inet static

Debian and Ubuntu use a single file to configure all the network interfaces

For more info run: man 5 interfaces

You can also break configuration into seperate files with:

source /etc/network/interfaces.d/*

Per interface configuration files give additional flexibility when using Chef, Puppet, Ansible or Salt

To put the configuration changes into effect you need to restart the network interface

Restarting the network interface: * ubuntu: initctl restart network-interface INTERFACE=interface * CentOS: systemctl restart network * Debian: systemctl restart networking

The same way linux treats many things as files, lnux treats many things as interfaces.


The interface is the basic building block of Linux Networking

VLAN interfaces are logical interfaces that allow an instance of Linux to communicate on multiple virtual local area networks (VLANs) simultaneously without having to have a dedicated physical interface for each VLAN

Create a VLAN (an extension of the ip link command):

ip link add link parent-device vlan-device type vlan id vlan-id
  • parent-device - physical adapter the logical VLAN interface is associated eg. eth1
  • vlan-device - name to be given to the logical VLAN interface (name of the parent device, a dot and then the VLAN ID eg. eth1.100
  • vlan-id - the 802.1Q VLAN ID value assigned to this logical interface

eg. This logical interface is to be associated with the physical interface named eth2 and should use 802.1Q VLAN ID 150

vagrant@jessie:~$ ip link add link eth2 eth2.150 type vlan id 150

Verify that the logical VLAN interface was added using ip link list. To verify (aside from the name) that the interface is a VLAN interface, add the -d parameter

vagrant@jessie:~$ ip -d link list eth2.150
7: eth2.150@eth2: <BROADCAST,MULTICAST> mtu 1500 qdisc noqueue state DOWN
   mode DEFAULT group default
   link/ether 00:0c:29:5f:d2:15 brd ff:ff:ff:ff:ff:ff
   vlan protocol 802.1Q id 150 <REORDER_HDR>

Finally the VLAN must be enabled:

ip link set eth2.150 up
ip addr add dev eth2.150

Just like physical interfaces, a logical VLAN interface that is enabled and has an IP address assigned will add a route to the host’s routing table

vagrant@jessie:~$ ip route list
default via dev eth0 dev eth0  proto kernel  scope link  src dev eth1  proto kernel  scope link  src dev eth2.150  proto kernel  scope link  src

To delete a VLAN interface

First disable the interface then remove the interface:

vagrant@jessie:~$ ip link set eth2.150 down
vagrant@jessie:~$ ip link delete eth2.150

VLAN interfaces will be tremendously useful anytime you have a Linux host that needs to communicate on multiple VLANs at the same time and you wish to minimize the number of switch ports and physical interfaces required

Routing as an End Host

A whole lot more stuff…

4. Learning Python in a Network Context

The network industry is fundamentally changing, there has never been a better time to learn to automate and write code

Things are starting to move in the right direction and the barrier to entry for network automation is more accessible than ever before

  • network device APIs
  • vendor- and community-supported Python libraries
  • freely available open source tools

Meaning less code, faster development and fewer bugs

Why Python?

  • Dynamically typed - create and use variables where needed where and when needed, no need to specify the data type.
  • It reads like a book
  • Many open source libraries and projects


In the book, the essentials of working with python is looked at. Most of this will be common to you if you have read any python tutorials or books.

One topic I needed a refresher on was: Passing Arguments to a Python Script

Passing Arguments to a Python Script

There is a module in python’s standard library to pass arguments from the command line into a python script. The module is called sys. Specifically we are using an attribute of the module called argv.

$ python 

sys.argv is a list of strings passed in from the Linux command line.

Contents of

#!/usr/bin/env python
import sys
if __name__ == '__main__':

Using argv we need to implement error handling. Additionally the user needs to know the precise order of inputs.

Python’s argparse module provides a way for user’s to enter arguments with flags.

Tips and Tricks

You should read through the tips and tricks available to you

One good one was python -i Which runs the script but then lets you interact with variable. Although import pdb; pdb.set_trace() was not discussed.

5. Data Formats and Data Models

In the same way that routers and switches require standardized protocols in order to communicate, applications need to be able to agree on some kind of syntax in order to exchange data between them…for this standard data formats are used like XML and JSON

Data models define how data in a format is structured

The goal of this chapter is to help you understand the value of standardized and simplified formats

XML: Not easy on the eye, but programmatically it is perfect

Types of Data

  • String - sequence of letter, numbers and symbols
  • Integer - a whole number (positive or negative)
  • Boolean - True or False
  • Advanced Data Structures - array, list, dicitonary.


  • Human Friendly
  • Represents data similar to XML and JSON but in a human readable way

Example: Represent a list of network vendors

- Cisco
- Juniper
- Brocade
- VMware

The --- at the top is a .yml convention indicating that the file is yaml.

In yaml you usually don’t need single quotes or double quotes to indicate a string. It is usually automatically discovered by the YAML parser (PyYaml)

Each item having a - in front of it. Meaning it is a list of 4 elements (as they do not have any under it)

YAML very closely mimics python data structures. A good example is mixing data types in a list:

- CoreSwitch
- 7700
- False
- ['switchport', 'mode', 'access']

In this example the the first item in the list is a string, the second is an integer, the third is a boolean and the fourth is a list. The first nested data structure!

Enclosing the 7700 in quotes: "7700" helps the parser figure out the data type. It is important to also enclose in quotes if the string contains a yaml special character like a :

Key value pairs:

Juniper: Also a plant
Cisco: 6500
Brocade: True
  - esxi
  - vcenter
  - nsx

Keys are the short strings to the left of the colons, the value is on the right.

YAML dictionaries can also be written in python like ways:

{Juniper: Also a plant, Cisco: 6500, Brocase: True, VMware: ['esxi', 'vcenter', 'nsx']}

Most parsers will see the above 2 as the same, yer the first one is much more readable

If you want it more human readable, use the more verbose options. With an API reabability is irrelevant so JSON or XML is preferred.


A # hash sign indicates a comment

- Cisco    # ocsiC
- Juniper  # repinuJ
- Brocade  # edacorB
- VMware   # erawMV

Reading Yaml with python

Install pyyaml:

pip install pyyaml

For example we create a file with the previous yaml content: example.yml:

import yaml

with open('example.yml', 'r') as file_:
    result = yaml.load(file_)

The output will be:

{'Juniper': 'Also a plant', 'Cisco': 6500, 'Brocase': True, 'VMWare': ['esxi', 'vcenter', 'nsx']}
<class 'dict'>

The with part is a contezt manager, that ensures the file is closed after use and it only available for the part of the program you need to use it for

Data Models in Yaml

The data model is the type of data expected, the blueprint for the data types.

Say we expected a key-value of manufacturer and device as strings:

Juniper: vSRX
Cisco: Nexus
Brocade: VDX

However, we got a different data model:

Juniper: Also a plant
Cisco: 6500
Brocade: True
  - esxi
  - vcenter
  - nsx

Valid Yaml, but invalid Data

  • Yaml does not have a built in data model description or validation mechanism
  • A reason why yaml is good for human to machine communication but not machine-to-machine


Comes with schema enforcement, transformations and advanced queries

lxml is the library of choice for dealing with xml with python

XML Basics

  • Hierachical by nature

    Cisco Nexus 7700 NXOS 6.1
  • <device> is called the root node
  • spacing and indentatino do not matter
  • children of <device> are <vendor>, <model>, <osver>

XML elements or nodes can also have attributes

<device type="datecenter-switch">

Namespaces can be used in xml to designate noes of the same name with different content and purpose

The xmlns designation is used for this:

    <e:device xmlns:c="">Palm Pilot</e:device>
    <n:device xmlns:m="">
        <n:model>Nexus 7700</n:model>
        <n:osver>NXOS 6.1</n:osver>

Using XML Schema Definition(XSD) for Data models

The XML Schema definition ensures the right kind of data is in a specific element

For the following example:

    <model>Nexus 7700</model>
    <osver>NXOS 6.1</osver>

We would write a schema to define what was expected:

<?xml version="1.0" encoding="utf-8"?>
<xs:schema elementFormDefault="qualified" xmlns:xs="
    <xs:element name="device">
            <xs:element name="vendor" type="xs:string"/>
            <xs:element name="model" type="xs:string"/>
            <xs:element name="osver" type="xs:string"/>
  • We define that each <device> element can have 3 children and each of these must contain a string
  • Some elements can be set as required

A python package called pyxb can be used to create a python file to represent this schema:

pip install PyXB

pyxbgen -u schema.xsd -m schema
  • pyxb still uses sourceforge for issues which is horrendous

I got this message, but the was created:

Python for AbsentNamespace0 requires 1 modules is an unreadable mess, but I think the point is how to use it (not how to read it):

In [1]: import schema
In [2]: device = schema.device()
In [3]: device.vendor = 'Cisco'
In [4]: device.model = 'Nexus'
In [5]: device.osver = '6.1'
In [10]: device.toxml(encoding='utf-8')
Out[10]: b'<?xml version="1.0" encoding="utf-8"?><device><vendor>Cisco</vendor><model>Nexus</model><osver>6.1</osver></device>'

More Info from w3c on schema defition

You can also use generateDS instead of pyxb

Tranforming XML with XSLT

Extensible Stylesheet Language Transformations (XSLT)

  • A template format.
  • A language for applying transformations to XML data

More info on XSLT from w3c

It is primarily used to convert XML into XHTML

Given this xml:

<?xml version="1.0" encoding="UTF-8"?>

we want to create an html table with the data, this is done with an XSLT document

<?xml version="1.0" encoding="UTF-8"?>

<xsl:stylesheet xmlns:xsl="" version="1.0">
<xsl:output indent="yes"/>
<xsl:template match="/">
    <table border="1">
    <tr bgcolor="#9acd32">
        <th style="text-align:left">First Name</th>
        <th style="text-align:left">Last Name</th>
    <xsl:for-each select="authors/author">
        <td><xsl:value-of select="firstName"/></td>
        <td><xsl:value-of select="lastName"/></td>
  • There is a for-each loop
  • The loop specifies the coordinate: authors/author of the XML document (called the xpath)
  • The value-of statement to dynamically insert a value as text from our xml data

So how do we get the HTML output:

In [1]: from lxml import etree
In [4]: xsl_root = etree.fromstring(open('table.xsl', 'rb').read())
In [5]: transform = etree.XSLT(xsl_root)
In [6]: xml_root = etree.fromstring(open('authors.xml', 'rb').read())
In [7]: trans_root = transform(xml_root)
In [9]: print(etree.tostring(trans_root))
b'<html><body><h2>Authors</h2><table border="1"><tr bgcolor="#9acd32"><th style="text-align:left">First Name</th><th style="text-align:left">Last Name</th></tr><tr><td>Jason</td><td>Edelman</td></tr><tr><td>Scott</td><td>Lowe</td></tr><tr><td>Matt</td><td>Oswalt</td></tr></table></body></html>'

Additional XSLT logic statements

  • <if> - only output if condition is met
  • <sort> - sorts before writing
  • <choose> - Advanced if statement (allows else if)

Network Configuration Example

XML data of interface data:

<?xml version="1.0" encoding="UTF-8"?>

XSLT for the router configuration:

<?xml version="1.0" encoding="UTF-8"?>
<xsl:stylesheet version="1.0" xmlns:xsl="">

<xsl:template match="/">
    <xsl:for-each select="interfaces/interface">
    interface <xsl:value-of select="name"/><br />
        ip address <xsl:value-of select="ipv4addr"/><br />


interface GigabitEthernet0/0
ip address
interface GigabitEthernet0/1
ip address
interface GigabitEthernet0/2
ip address

It is good but also a bit cumbersome

Searching XML with XQuery

XQuery helps extract data from an XML document

Further reading on XQuery can be found on w3c



  • Network Programmability and Automation - Jason Edelman, Scott S. Lowe, Matt Oswalt