YANG

YANG (Yet Another Next Generation) (RFC 7950) is a data modeling language used to programmatically represent both configuration and operational state data on network devices. It is defined in RFC 6020 and serves as the foundation for modern, model-driven network management and automation.

Purpose and Benefits

YANG promotes unification in networking by enabling structured, programmatic interfaces to devices (e.g., routers and switches) using protocols like NETCONF, RESTCONF, or gNMI. It replaces traditional unstructured CLI outputs or legacy protocols like SNMP with vendor-neutral, hierarchical data models that map directly to programmable entities (like JSON structures). Key advantages include:

  • Vendor-neutral modeling for consistency across devices.
  • Structured data for easier automation and integration with tools/languages.
  • Extensibility through augmentations and deviations.
  • Better support for DevOps/NetDevOps workflows (e.g., Ansible, pyATS, Terraform).

How YANG Models Work

YANG organizes data hierarchically as a tree, where each element is a node with a name and either a value or child nodes. Models are written in modules, which include:

  • Header statements (name, namespace, prefix, imports).
  • Revision statements (version history).
  • Definition statements (the core data model).

YANG defines four main node types:

  • Leaf : A single value (no children), e.g., interface name (string) or enabled status (boolean).
  • Leaf-list : An ordered sequence of leaf values.
  • Container : Groups related nodes (no value itself); can be configuration (config true, read-write) or operational state (config false, read-only).
  • List : A sequence of entries (like a list of interfaces), each identified by a key (e.g., interface name).

Note: A model is an abstract concept, while a module is the realization of that concept. In practice, however, these two terms are often used interchangeably.

Standard vs. Vendor-Specific Models

  • Industry-standard models : Developed by IETF (e.g., ietf-interfaces.yang, ietf-ip.yang) or OpenConfig for broad compatibility.
  • Vendor-specific (native) models : Tailored to a specific platform. These are often required when standard models lack coverage for vendor-unique features.

All models are publicly available on GitHub:

  • https://github.com/YangModels/yang/
  • https://github.com/openconfig/public/.

Key Features: Augmentations and Deviations

  • Augmentations : Extend base models (e.g., ietf-ip.yang adds IPv4/IPv6 details to ietf-interfaces.yang using augment and conditional when statements).
  • Deviations : Handle implementation differences (e.g., marking a feature as not-supported in a specific OS release).

Practical Mapping and Tools

YANG structures map naturally to programming concepts, especially Python:

YANG Element Python Equivalent
Container Dictionary
Leaf name Dictionary key
Leaf Dictionary value
List List
String/Boolean/Integer Corresponding types
Empty None

Tools like Pyang (pip install pyang) let you validate models and visualize them as trees (e.g., pyang -f tree ietf-interfaces.yang).