Domain 2 of 4 · Chapter 4 of 7

Network Simulation with Cisco CML

Unlock the complete study guide + 1,040 practice questions across 16 full exams.

Bundled into the existing Designing, Deploying and Managing Network Automation Systems premium course — no separate purchase.

14-day money-back guarantee — no questions asked.

Included in this chapter:

  • Why boot a real NOS, not a diagram
  • A lab is nodes, links, and one YAML file
  • The node lifecycle: DEFINED, STARTED, BOOTED, reset
  • Make nodes reachable: day-0 config and the connector
  • Two planes: the API, virl2_client, and where config goes
  • Test automation in an ephemeral CI lab
  • Reading the 350-901 stem

Where a change gets validated

PropertyStatic + dry-runCML simulationProduction canary
Runs real NOS imagesNoYesYes
Writes real device configNoYes (to the sim)Yes (live subset)
Catches NOS-version errorsNoYesYes
Blast radius of a bugNoneNone (disposable lab)Limited live subset
Typical pipeline stageprevalidationprevalidation / pre-mergedeploy (canary)

Decision tree

Needs real NOS behavior? not just valid syntax No Static + dry-run prevalidation stage Yes Image + topology in CML? is the NOS available Yes Ephemeral CML lab boot, test, wipe No Limited blast radius OK? on a live subset Yes Production canary last resort No Add image to CML then test in a lab
Cheat sheet

Cheat sheet

  • A CML lab is nodes joined by links, stored as YAML
  • Node states: DEFINED, STARTED, BOOTED
  • Node definitions set the NOS and interface naming
  • External-connector bridges the lab to the outside network
  • CML is a disposable replica for pre-prod testing
  • Point a pyATS testbed at CML nodes
  • CI drives an ephemeral CML lab per run
  • Day-0 config seeds management reachability
  • CML exposes a token-authenticated REST API
  • virl2_client scripts the lab lifecycle
  • Automation uses the management IP, not the console

Unlock with Premium — includes all practice exams and the complete study guide.

References

  1. Ansible Playbook Check Mode (dry run)
  2. Cisco Modeling Labs Documentation (CML 2.x)
  3. Cisco Modeling Labs FAQ FAQ
  4. Importing Labs (YAML topology files)
  5. Creating a New Node Definition
  6. Starting, Stopping, and Wiping Nodes
  7. Starting Simulations (boot timeout)
  8. Node Configuration Files (day-0 / bootstrap)
  9. Networking External Connectors
  10. External Connector NAT Mode (default)
  11. System Bridge Mode
  12. CML User Authentication
  13. virl2_client (CML 2.x Client Library)
  14. pyATS Documentation