Why I built netlld: network design docs that don't go stale

Every data center fabric I have worked on needed a Low-Level Design. And on every one of them, the LLD had the same two problems: it took days to write, and it started going out of date the moment the network changed.

The document describes the network you meant to build. The network keeps changing anyway.

The problem with hand-written designs

An LLD is only useful if people trust it. Once an engineer finds one wrong VLAN or one missing link, they stop reading the document and start logging into switches instead. From then on it's a file nobody updates, because nobody uses it.

Writing it by hand also means the same facts get typed several times: once in the design, again in the bill of materials, again in the statement of work and the project plan. Each copy can drift from the others.

Generate it from the network instead

netlld takes an Arista EVPN/VXLAN fabric and produces the design from the network itself. It reads Arista AVD data, or just the switches' running-configs, and builds one vendor-neutral model of the fabric. Everything else comes from that one model:

the Low-Level Design document
physical, underlay and overlay topology diagrams
a design review with findings
a bill of materials, statement of work and project plan

netlld system diagram: inputs feed one fabric model, which generates the LLD, diagrams, findings, BOM, SOW and project plan. — One fabric model in the middle. Every input turns into it, and every output comes from it.

Three things I cared about most

It works on fabrics with no automation

Most real networks weren't built with AVD. So netlld can rebuild the design from plain running-configs, and a test checks that the result matches real Arista AVD output.

It goes back to AVD

A design you can't deploy is just a picture. netlld exports a runnable Ansible/AVD project that keeps the deployed addressing, verified end to end against arista.avd 6.3.0.

It keeps checking

Drift detection compares the design with the live network over eAPI or CloudVision, and CI rebuilds and republishes the document on every change. That's what makes it a living document instead of a snapshot.

Where it is now

netlld is in an invite-only preview, backed by 525 automated checks. If you run Arista EVPN/VXLAN fabrics, I'd love to hear what you would use it for.

Independent project, built and maintained by me. Not affiliated with Arista Networks or my employer. All sample data is synthetic.