YANG Data Structure

YANG (Yet Another Next Generation) is a data modeling language used to describe the structure and configuration of network devices. A data model is a well understood and agreed upon method to describe something.

Data models may describe some aspect of a network device, or a network service. Network servies like Interface, vlan,OSPF.. and Network services like VRF, ACL..

It does not configure devices directly, but defines how configuration and state data are structured, so tools like NETCONF / RESTCONF /grpc can read or modify them in a standardized way.

Simple YANG Example

module simple-interface {
  namespace "urn:example:interface";
  prefix if;

  container interfaces {
    list interface {
      key "name";

      leaf name {
        type string;
      }

      leaf description {
        type string;
      }

      leaf enabled {
        type boolean;
        default true;
      }

      leaf ip-address {
        type string;
      }
    }
  }
}

This means: A device has interfaces, Each interface has: name, description, enabled / disabled and IP address.

You can find the Yang data model here: https://github.com/YangModels/yang

There are standard and also vendor base Yang data models.

Core YANG Concepts

A module is the top-level file in YANG. Everything lives inside a module. Think of it as: A project folder that contains everything related to this model.

module router-config {
  namespace "urn:example:router";
  prefix rt;

A container groups related configuration items. For example here this container holds all interface-related data.

container interfaces {

A list is used when something can appear multiple times(e.g., multiple interfaces). GigabitEthernet0/0, GigabitEthernet0/1, Loopback0, Each one is an entry in this list.

list interface {
  key "name";

A leaf holds one value only. for example name = "GigabitEthernet0/0", ip-address = "192.168.1.1" ,enabled = true

leaf name {
  type string;
}

leaf ip-address {
  type string;
}

leaf enabled {
  type boolean;
}

A leaf-list holds multiple values of the same type. Unlike list, it has no keys, only values.

leaf-list dns-servers {
  type string;
}

typedef is used to define reusable data types.

typedef ipv4-address {
  type string {
    pattern "([0-9]{1,3}\\.){3}[0-9]{1,3}";
  }
}

rpc Remote Procedure Call is for actions, not configuration. For example Reset interface, clear counters..

rpc restart-interface {
  input {
    leaf name {
      type string;
    }
  }
}

Full Example:
module router-config {
  namespace "urn:example:router";
  prefix rt;

  typedef ipv4-address {
    type string;
  }

  container interfaces {
    list interface {
      key "name";

      leaf name {
        type string;
      }

      leaf ip-address {
        type ipv4-address;
      }

      leaf enabled {
        type boolean;
      }
    }
  }

  rpc restart-interface {
    input {
      leaf name {
        type string;
      }
    }
  }

  notification interface-down {
    leaf name {
      type string;
    }
  }
}

We can also open a yang file with pyang in linux, so we can have a better look:

$pyang -f tree router-config.yang
module: router-config
  +--rw interfaces
     +--rw interface* [name]
        +--rw name          string
        +--rw ip-address   ipv4-address
        +--rw enabled      boolean
  +---x restart-interface
     +--rw input
        +--rw name   string
  +---n interface-down
     +--rw name   string

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