Network Simulation and Test Tools: CML and pyATS

Mental model

Cisco objective 5.3 says “Describe the use and roles of network simulation and test tools (such as Cisco Modeling Labs and pyATS)”. Two tools, one answers “where do I get a Cisco network to test against”, the other answers “how do I automate tests against it”.

Note: the v1.1 blueprint replaced “VIRL” with “Cisco Modeling Labs (CML)”. Same product family; VIRL was the old marketing name.

CML (Cisco Modeling Labs)

A virtual environment that runs real Cisco OS images.

• Nodes = devices (IOS XE router, NX-OS switch, ASA firewall, external connector, Ubuntu VM).
• Links = virtual cables between node interfaces.
• Launch a topology; CML boots each node (takes seconds-minutes); you SSH / console into them and configure them like real gear.
• Export / import topologies as YAML or XML.
• API: REST, so you can terraform apply to spin up a topology, run tests, tear it down.

Why you use it

• Learn / practice without buying hardware.
• Reproduce bugs reported by prod.
• CI pipeline: spin up a mini topology, apply the config change, run smoke tests, tear down.
• Design validation before buying real gear.

Flavours

• CML Personal — $199/year, free for CCNA/CCNP/DevNet certified. Up to 20 nodes.
• CML Enterprise — larger (up to 160 nodes) and runs on a server.
• DevNet Sandbox has free always-on CML topologies.

pyATS (Python Automated Test System)

Cisco’s open-source Python framework for network testing. Pronounced “py-ATS” or sometimes “py-A-T-S”.

What a pyATS test looks like

from pyats import aetest

class VerifyIntfs(aetest.Testcase):

    @aetest.setup
    def connect(self, testbed):
        self.device = testbed.devices["sw1"]
        self.device.connect()

    @aetest.test
    def ge0_0_is_up(self):
        out = self.device.parse("show ip interface brief")
        assert out["interface"]["GigabitEthernet0/0"]["status"] == "up"

• Describes test cases with decorators (@aetest.setup, @aetest.test, @aetest.cleanup).
• Uses a testbed (YAML inventory) to know how to connect to devices.
• device.parse("show ...") returns a dict, powered by Genie parsers — pre-written parsers for hundreds of Cisco show commands.
• pyats run job myjob.py runs the suite, generates HTML report.

Why you use it

• Automated regression tests: “every day, confirm OSPF has these 50 neighbors, no interface errors are counting up, STP root is sw-core-1”.
• Pre/post change verification in CI: “before this change, these 500 facts were true; apply the change; are they still true?”.
• Load / scale tests on CML topologies.

CML + pyATS in a CI pipeline

Common pattern:

1. Developer opens PR with new network config.
2. CI pipeline uses CML API to spin up a topology.
3. Apply the proposed config to the virtual devices.
4. Run pyATS against them: convergence, reachability, counters.
5. If all tests pass, promote; otherwise fail the PR.
6. Tear down the CML topology.

Alternative tools (good to know but not on 200-901)

• GNS3 — open-source, similar concept, supports more images (including some non-Cisco).
• EVE-NG — community favourite for mixed-vendor labs.
• Containerlab — recent, container-based (nokia SR Linux, Cisco XRd, Arista cEOS), fast to spin up.
• Batfish — static analysis of configs; catches issues without actually booting anything.

FAQ

Do I need CML to pass 200-901? No. The exam tests understanding. For practice, you can rely on DevNet Sandbox free topologies.

Does CML run real Cisco software? Yes — real IOS XE, NX-OS, ASA, IOS XRv images. That is the whole point vs cheaper mocks.

What is Genie? A pyATS add-on that turns show-command output into structured Python dicts. Covers hundreds of commands across IOS XE, NX-OS, Junos, more. Hugely reduces the “parse free text” grief.

Is pyATS only for test? Can I run config too? Yes to both. pyATS exposes device.configure() and device.execute(). Many teams use it as both test runner AND lightweight automation driver.