Container nodes in Cisco Modeling Labs (CML) 2.9 complement digital machines, providing higher flexibility and effectivity. Engineers profit from having light-weight, programmable, and quickly deployable choices inside their simulation environments. Whereas digital machines (VMs) dominate with community working methods, containers add flexibility, enabling instruments, visitors injectors, automation, and full purposes to run easily along with your CML topology. Conventional digital machines are nonetheless efficient, however customized containers introduce a transformative agility.
Constructing pictures that behave predictably and combine cleanly with simulated networks is way simpler with containers. As anybody who has tried to drop a inventory Docker picture into CML rapidly discovers, this isn’t a simple course of. Typical Docker pictures lack the mandatory CML-compatible metadata, community interface behaviors, and lifecycle properties. Utilizing containers with CML is the lacking aspect.
This weblog submit supplies a sensible, engineering-first walkthrough for constructing containers which can be really CML-ready.
Be aware about enhancements to CML: When containers had been launched, just one picture per node definition was allowed. With the CML 2.10 launch, this restriction has been lifted. Particularly, the next enhancements shall be added:
- Per picture definition, Docker tag names akin to:
debian:bookworm, debian:buster and debian:trixie
Are all legitimate tags for a similar “debian-docker” node definitions—three legitimate picture definitions for one node definition.
- Specification of Docker tags as an alternative choice to picture names (.tar.gz recordsdata) and SHA256 has sums. On this case, CML will attempt to obtain the picture from a container registry, e.g., Docker Hub, if not in any other case specified.
- Improved launch logic to keep away from “perpetual launches” in case the SHA256 sum from the picture definition didn’t match the precise hash sum within the picture.
Why do customized containers in CML matter?
Conventional CML workflows depend on VM-based nodes working IOSv, IOS-XRv, NX-OS, Ubuntu, Alpine, and different working methods. These are glorious for modeling community working system habits, however they’re heavyweight for duties akin to integrating CLI instruments, net browsers, ephemeral controllers, containerized apps, microservices, and testing harnesses into your simulations.
Containers begin rapidly, devour fewer sources, and combine easily with commonplace NetDevOps CI/CD workflows. Regardless of their benefits, integrating commonplace Docker pictures into CML isn’t with out its challenges, every of which requires a tailor-made resolution for seamless performance.
The hidden challenges: why a Docker picture isn’t sufficient
CML doesn’t run containers in the identical means a vanilla Docker Engine does. As an alternative, it wraps containers in a specialised runtime surroundings that integrates with its simulation engine. This results in a number of potential pitfalls:
- Entry factors and init methods
Many base pictures assume they’re the solely course of working. In CML, community interfaces, startup scripts, and boot readiness ought to be supplied. Additionally, CML expects a long-running foreground course of. In case your container exits instantly, CML will deal with the node as “failed.” - Interface mapping
Containers usually use eth0, but CML attaches interfaces sequentially primarily based on topology (eth0, eth1, eth2…). Your picture ought to deal with further interfaces added at startup, mapping them to particular OS configurations. - Capabilities and customers
Some containers drop privileges by default. CML’s bootstrap course of may have particular entry privileges to configure networking or begin daemons. - Filesystem format
CML makes use of non-obligatory bootstrap property injected into the container’s filesystem. A normal Docker picture received’t have the precise directories, binaries, or permissions for this. If wanted, CML can “inject” a full suite of command-line binaries (“busybox”) right into a container to supply a correct CLI surroundings. - Lifecycle expectations
Containers ought to output log data to the console in order that performance could be noticed in CML. For instance, an online server ought to present the entry log.
Misalign any of those, and also you’ll spend hours troubleshooting what seems to be a easy “it really works with run” situation.
How CML treats containers: A psychological mannequin for engineers
CML’s container capabilities revolve round a node-definition YAML file that describes:
- The picture to load or pull
- The bootstrap course of
- Atmosphere variables
- Interfaces and the way they bind
- Simulation habits (startup order, CPU/reminiscence, logging)
- UI metadata
When a lab launches, CML:
- Deploys a container node
- Pulls or masses the container picture
- Applies networking definitions
- Injects metadata, IP handle, and bootstrap scripts
- Displays node well being by way of logs and runtime state
Consider CML as “Docker-with-constraints-plus-network-injection.” Understanding CML’s strategy to containers is foundational, however constructing them requires specifics—listed below are sensible suggestions to make sure your containers are CML-ready.
Ideas for constructing a CML-ready container
The container pictures constructed for CML 2.10 and ahead are created on GitHub. We use a GitHub Motion CI workflow to totally automate the construct course of. You’ll be able to, in actual fact, use the identical workflow to construct your individual customized pictures able to be deployed in CML. There’s loads of documentation and examples you could construct off of, supplied within the repository* and on the Deep Wiki.**
Vital observe: CML treats every node in a topology as a single, self-contained service or utility. Whereas it is likely to be tempting to instantly deploy multi-container purposes, usually outlined utilizing docker-compose , into CML by trying to separate them into particular person CML nodes, this strategy is mostly not really useful and might result in vital issues.
1.) Select the precise base
Begin from an already present container definition, like:
- nginx (single-purpose community daemon utilizing a vanilla upstream picture).
- Firefox (graphical consumer interface, customized construct course of).
- Or a customized CI-built base along with your commonplace automation framework.
Keep away from utilizing pictures that depend on SystemD until you explicitly configure it; SystemD inside containers could be difficult.
2.) Outline a correct entry level
Your container should:
- Run a long-lived course of.
- Not daemonize within the background.
- Assist predictable logging.
- Hold the container “alive” for CML.
Right here’s a easy supervisor script:
#!bin/sh echo "Container beginning..." tail -f /dev/null
Not glamorous, however efficient. You’ll be able to substitute tail -f /dev/null along with your service startup chain.
3.) Put together for a number of interfaces
CML might connect a number of interfaces to your topology. CML will run a DHCP course of on the primary interface, however until that first interface is L2-adjacent to an exterior connector in NAT mode, there’s NO assure it’s going to purchase one! If it can not purchase an IP handle, it’s the lab admin’s accountability to supply IP handle configuration per the day 0 configuration. Usually, ip config … instructions can be utilized for this goal.
Superior use circumstances you possibly can unlock
When you conquer customized containers, CML turns into dramatically extra versatile. Some common use circumstances amongst superior NetDevOps and SRE groups embrace:
Artificial visitors and testing
Automation engines
- Nornir nodes
- pyATS/Genie check harness containers
- Ansible automation controllers
Distributed purposes
- Primary service-mesh experiments
- API gateways and proxies
- Container-based middleboxes
Safety instruments
- Honeypots
- IDS/IPS elements
- Packet inspection frameworks
Deal with CML as a “full-stack lab,” enhancing its capabilities past a mere community simulator.
Make CML your individual lab
Creating customized containers for CML turns the platform from a simulation instrument into an entire, programmable check surroundings. Whether or not you’re validating automation workflows, modeling distributed methods, prototyping community features, or just constructing light-weight utilities, containerized nodes let you adapt CML to your engineering wants—not the opposite means round.
When you’re prepared to increase your CML lab, the easiest way to start out is easy: construct a small container, copy and modify an present node definition, and drop it right into a two-node topology. When you see how easily it really works, you’ll rapidly understand simply how far you possibly can push this characteristic.
Would you wish to make your individual customized container for CML? Tell us within the feedback!
* Github Repository – Automation for constructing CML Docker Containers
** DeepWiki – CML Docker Containers (CML 2.9+)
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