📘Cisco DevNet Associate (200-901 DEVASC)

Modern network automation requires a safe environment to build, test, and validate network configurations, APIs, and automation scripts before deploying them in production networks. Network simulation and testing tools help developers and network engineers design, test, troubleshoot, and validate network behavior without affecting real infrastructure.

Two important tools covered in the Cisco DevNet Associate (200-901 DEVASC) exam are:

Cisco Modeling Labs (CML)
pyATS

These tools play different but complementary roles in network development, testing, and automation validation.

1. Why Network Simulation and Testing Tools Are Important

In modern IT environments, networks are often automated using APIs, scripts, and infrastructure-as-code tools. If changes are applied directly to production networks without testing, they can cause:

Network outages
Misconfigurations
Security vulnerabilities
Service interruptions

Network simulation and testing tools solve this problem by allowing engineers to:

Build virtual networks
Test automation scripts
Validate network configurations
Simulate real network devices
Run automated verification tests

Key Benefits

Benefit	Description
Safe Testing	Test configurations without affecting production
Automation Validation	Ensure scripts work correctly
Faster Development	Developers can test code quickly
Troubleshooting	Recreate issues in a lab environment
Learning	Students can practice networking without real hardware

These tools are widely used in DevNet workflows, CI/CD pipelines, and network automation testing.

2. Network Simulation Tools

Network simulation tools allow engineers to create a virtual network environment that behaves like real hardware devices.

They simulate devices such as:

Routers
Switches
Firewalls
Servers

This enables developers to build entire network topologies in software.

3. Cisco Modeling Labs (CML)

What is Cisco Modeling Labs?

Cisco Modeling Labs is a network simulation platform that allows users to design and run virtual network topologies using Cisco operating systems.

It runs Cisco network images such as:

IOSv
IOS XRv
NX-OSv
ASA
Linux nodes

This allows users to create realistic Cisco network environments without physical hardware.

How Cisco Modeling Labs Works

CML runs on:

Local servers
Virtual machines
Cloud environments

Users create networks by:

Opening the CML web interface
Dragging network devices into a topology
Connecting them with virtual links
Starting the simulation

Once running, users can:

Access device CLI
Configure routers and switches
Test network protocols
Run automation scripts

Example IT Use Case

A network engineer wants to test an automation script that configures OSPF routing across multiple routers.

Instead of using real devices, they:

Build a virtual topology of routers in CML
Run the automation script using Python
Verify the routing tables
Confirm OSPF neighbors form correctly

If something fails, the engineer can debug safely without affecting real networks.

Key Features of Cisco Modeling Labs

1. Virtual Network Topology Creation

Users can create complex network topologies including:

Data center networks
Enterprise networks
Multi-router environments

Example topology:

Router1 ----- Router2
   |             |
 Switch1       Switch2

All devices are virtual.

2. Real Cisco Operating Systems

Unlike simple simulators, CML runs actual Cisco virtual operating systems, which behave like real devices.

This provides high-accuracy testing environments.

3. API Access

CML provides a REST API, allowing automation tools to:

Create topologies automatically
Start and stop simulations
Retrieve network information

This is useful for automation pipelines.

4. Integration with Automation Tools

CML can integrate with tools such as:

Python scripts
Ansible
CI/CD pipelines
Testing frameworks

Developers can automatically spin up test networks during software testing.

Typical Use Cases for CML

Use Case	Description
Network design testing	Validate topology design
Automation testing	Run scripts against simulated devices
Training	Practice networking skills
Troubleshooting	Recreate network problems
Development labs	Build environments for DevNet labs

4. Network Testing Tools

While simulation tools build networks, testing tools validate network behavior and configurations.

These tools ensure that networks operate as expected after:

Configuration changes
Automation deployments
Software updates

5. pyATS

What is pyATS?

pyATS is a Python-based network testing and automation framework developed by Cisco.

It is used to:

Automate network testing
Validate network state
Perform configuration verification
Run automated test cases

pyATS is widely used in network automation and DevOps pipelines.

Components of pyATS

pyATS consists of several key components.

1. Testbed

A testbed describes the network devices used for testing.

It is usually defined in a YAML file.

Example:

devices:
  router1:
    os: iosxe
    type: router
    connections:
      cli:
        protocol: ssh
        ip: 10.1.1.1

This tells pyATS how to connect to the device.

2. Test Scripts

Test scripts define automated tests written in Python.

These scripts perform tasks such as:

Running show commands
Checking interface status
Validating routing tables

Example workflow:

Connect to device
Run show ip interface brief
Verify interfaces are up
Generate test result

3. Genie Libraries

Genie is a library within pyATS that helps parse and validate network data.

Example:

Raw CLI output:

show ip route

Genie converts it into structured data like:

{
 "routes": {
  "10.1.1.0/24": {
   "protocol": "ospf"
  }
 }
}

This allows scripts to easily analyze network information.

4. Automated Test Execution

pyATS runs automated tests that produce:

Pass results
Fail results
Detailed reports

Example report output:

Test ResultsTestcase: Interface Status
Result: PASSTestcase: OSPF Neighbor Check
Result: FAIL

6. Example IT Workflow Using pyATS

Example: Verifying a network after configuration deployment.

Step 1 — Deploy configuration using automation tools.

Step 2 — Run pyATS tests to verify:

Interfaces are operational
Routing protocols are working
VLANs are configured correctly

Step 3 — Generate automated report.

If tests fail, the deployment can be rolled back before users experience problems.

7. Integration with DevOps and CI/CD

Network testing tools like pyATS are commonly integrated into CI/CD pipelines.

Typical workflow:

Developer writes automation script
Simulation environment is created (CML)
Script configures devices
pyATS runs verification tests
Pipeline reports success or failure

This ensures network changes are validated automatically before deployment.

8. Differences Between CML and pyATS

Feature	Cisco Modeling Labs	pyATS
Type	Simulation tool	Testing framework
Purpose	Create virtual networks	Test network behavior
Environment	Virtual topology	Real or simulated devices
Usage	Network design and simulation	Automation testing
Interface	Web UI and API	Python scripts

Simple summary

CML → Build the virtual network
pyATS → Test the network automatically

They are often used together in automation workflows.

9. Role in Network Automation Development

Network simulation and testing tools play important roles in modern infrastructure automation.

Development Phase

Developers write automation scripts.

Simulation Phase

CML creates a virtual network for testing.

Testing Phase

pyATS runs automated verification tests.

Deployment Phase

Validated automation is deployed to production.

This approach ensures reliable, safe, and repeatable network automation.

10. Key Exam Points (DEVASC 200-901)

For the exam, remember the following:

Cisco Modeling Labs (CML)

Cisco network simulation platform
Creates virtual network topologies
Runs virtual Cisco devices
Used for design, testing, and training
Provides API integration

pyATS

Python-based network testing framework
Used for automated validation
Supports testbed definitions
Uses Genie parsers for structured data
Generates automated test reports

Combined Workflow

CML + pyATS enables:

Simulation
Automation testing
Continuous network validation

Final Summary

Network simulation and testing tools are essential in modern network automation workflows.

Cisco Modeling Labs (CML) allows engineers to build virtual network environments that simulate real Cisco devices. It is used for topology design, automation testing, and network experimentation.
pyATS is a Python-based automated network testing framework that validates device configurations, verifies network operations, and generates detailed test reports.

Together, these tools help developers and network engineers build reliable automation, test network behavior safely, and prevent configuration errors before deployment.