Interpret basic YANG models

📘Cisco DevNet Associate (200-901 DEVASC)

---

1. What is YANG?

• YANG stands for Yet Another Next Generation.
• It is a data modeling language used to describe network device configurations and state data in a structured way.
• Think of it as a blueprint or schema that defines what data a network device can store and how it is organized.
• YANG is mainly used with NETCONF and RESTCONF to automate network management.

Key idea: YANG is not the actual data, it describes the structure of the data that can be sent to or received from network devices.

---

2. Why YANG is Important in Networking

• Allows automation tools (like Python scripts, Ansible, or Cisco DNA Center) to understand what data they can configure or read from a device.
• Provides consistency in how devices represent data.
• Makes it easier to validate and check network configurations automatically.

Example in IT: A YANG model can define the exact structure for a router interface configuration:

• Interface name
• IP address
• Subnet mask
• Status (up/down)

Automation tools will read the YANG model to know exactly what fields are required and what data is allowed.

---

3. YANG Data Model Structure

YANG models define data in a hierarchical (tree-like) structure, similar to folders and files on a computer.

Key Components of a YANG Model

1. Module
   • A YANG file starts with a module.
   • The module gives a name and version for the set of configurations it defines.
   • Example: module cisco-interfaces { ... }
2. Container
   • A container groups related data together (like a folder for related settings).
   • Example: interfaces container holds all interface configurations.
3. Leaf
   • A leaf is an individual piece of data (like a file in a folder).
   • Example: name, ip-address, description.
4. Leaf-list
   • A list of multiple leaves with the same type.
   • Example: multiple DNS server IP addresses for a router.
5. List
   • A list contains multiple containers of the same type.
   • Example: a list of interfaces, where each interface has name, IP, and status.
6. Choice and Case
   • Provides optional alternatives.
   • Example: an interface can have either a static IP configuration or a DHCP configuration.
7. Type
   • Defines the data type for a leaf or leaf-list.
   • Examples: string, int32, boolean, enumeration.
8. Augment
   • Allows extending existing models with additional data.
   • Useful when you need extra configurations not originally defined.

---

4. Understanding YANG through a Device Configuration Example

Imagine you want to configure interfaces on a network switch.

YANG Model (simplified):

module cisco-interfaces {
  container interfaces {
    list interface {
      leaf name { type string; }
      leaf ip-address { type string; }
      leaf status { type enumeration { enum up; enum down; } }
    }
  }
}

• Module: cisco-interfaces → defines interface settings.
• Container: interfaces → groups all interfaces.
• List: interface → multiple interfaces allowed.
• Leaf: name, ip-address, status → actual settings for each interface.
• Type: string or enumeration → defines allowed values.

Automation tools can read this model and know:

1. You must provide a name for the interface.
2. Status can only be up or down.
3. Multiple interfaces can be configured in a list.

---

5. Key YANG Concepts for the Exam

Concept	What it Means	Example in IT
Module	A YANG file containing a set of configurations	cisco-interfaces
Container	Groups related configurations	interfaces container
Leaf	Single data element	ip-address
Leaf-list	Multiple entries of the same type	Multiple DNS servers
List	A collection of containers	Multiple router interfaces
Type	Defines data type	string, boolean, enumeration
Choice/Case	Optional alternatives	Static IP OR DHCP
Augment	Extending an existing model	Adding extra QoS settings

---

6. How YANG is Used in IT Automation

• NETCONF and RESTCONF use YANG models to interact with devices.
• Automation tools read YANG models to:
  • Know which fields are required.
  • Validate configuration data.
  • Ensure the data format is correct before sending it to the device.

Example workflow:

1. RESTCONF request is made to configure an interface.
2. YANG model defines which data is allowed (name, ip-address, status).
3. Automation tool checks data against YANG model.
4. Configuration is applied if valid, or rejected if invalid.

---

7. Tips for the Exam

• Remember: YANG describes data structure, not actual configuration.
• Know common YANG statements: module, container, list, leaf, type.
• Understand hierarchical structure of YANG models.
• Be able to interpret small YANG snippets to identify what data is defined and required.
• Focus on interfaces, IP addresses, and status fields — these are commonly used examples in the exam.

---

✅ Summary:

• YANG is the blueprint for network configurations.
• It helps automation tools understand and validate device data.
• Key elements: module, container, list, leaf, leaf-list, type, choice, augment.
• Being able to read and interpret YANG models is essential for automation and network programmability tasks.