What are YANG models and why should I care?
Models are structured representations of network elements and their associated configured and operational state.
YANG (Yet Another Next Generation - RFC6020) is a data modeling language. YANG is used to describe the configured and operational state of network elements. This allows for the consistent and structured representation of device and protocol attributes. The model definition typically includes the low level data types used to represent operational attributes as well as constraints that should be enforced in configuration.
These modeled representations provide a consistent mechanism for device configuration as well as validating the device state through streaming telemetry or other device interrogation methods. These modeled device representations can also be transformed into data structures that can be used by configuration systems to dynamically control device configuration or state.
YANG has been adopted as the official modeling language of the IETF and the OpenConfig group.
The for the past few years the IETF has been providing models for previously standardized technologies as well as protocols and technologies which are under active standardization. These models are commonly used with complementary IETF standardized network management protocols (for example, NETCONF and RESTCONF) and have been selectively incorporated into other modeling initiatives.
OpenConfig is an operator led group defining models for network devices. Their models have been widely adopted by a number of vendors including Arista and an ecosystem of open source tooling has emerged to utilize these models for telemetry and configuration applications.
In addition to defining device and protocol models the OpenConfig group has defined complementary network management protocols which leverage these models for telemetry (gNMI), configuration (gNMI), operational activities (gNOI) and RIB manipulation (gRIBI). Though it bears noting that OpenConfig models can also be used with NETCONF and RESTCONF protocols as well.
Details regarding the OpenConfig model support within Arista's EOS software can be found online.
While standard models provide a wide range of feature coverage there is commonly a need to model vendor-specific device or feature operation. Examples of this may include device specific behaviors (hardware configuration details) or pre-standard functionality that is deployed in operator networks that needs to be managed using the same tooling.
Vendors may opt to define all new models which are published independently. Alternately, vendors may choose to leverage an existing standardized model and add vendor-specific elements to the standard models these are known as model augments. These augments are commonly published to allow integration into operator tooling.
Arista publishes augments associated with a given EOS release on GitHub.