YAML Basics for DevNet

October 21, 2019 By Rowell 1 Comment

Taking a look at the Cisco DevNet Associate exam topics (DEVASC 200-901), one of the first objectives is to compare different data formats. Those data formats you should be able to compare are XML, JSON, and YAML.

To start, I decided to take a look at YAML and what it has to offer from a very basic overview. I’m very new when it comes to DevOps or NetDevOps but this will become my notes of sorts as I look into the DevNet Associate certification.

YAML is a human-readable structured data format which stands for Stands for Yet Another Markup Language or now YAML Ain’t Markup Language, depending on where you look The official documentation page states the latter.

Goals of YAML

• Data serialization standard for programming languages.
• Human-readable structured data format.
• Portable between programming languages.
• Consistent model.
• Expressive and extensible.
• Easy to implement and use.

A quick overview of YAML

• Writable and readable.
• Case sensitive
• Files end in .yaml or .yml
• Spaces are used instead of tabs (because various tools treat tabs differently)
• Indentation is used for structure
• Colons separate key-value pairs
• Dashes create bullet lists
• The start of a YAML file begins with three dashes

Data structures:

• Mappings (hashes/dictionaries)
• Sequences (arrays/lists)
• Scalars (strings/numbers)

Working with Blocks
Block collections use indentation for scope and begin each entry on its new line. Block sequences use a dash and space to indicate each entry. Mappings use a colon and space to mark a key-value pair.

To create a comment, you must use a hash tag.

Sequence
A sequence entry will be denoted by a dash -.

 - netsw-idf-01
 - netsw-idf-02
 - netsw-idf-03 

Scalar
A scalar is just a single value. They can be strings, numbers, Boolean, or null values.

# scalars can be quoted
“GigabitEthernet1/0/1”

# scalars can also be unquoted
GigabitEthernet1/0/2 

Mapping Scalars to Sequences
Also creates an array.

routers:
    - rtr-idf-01
    - rtr-idf-02
switches:
    - netsw-idf-01
    - netsw-idf-02

Sample Inventory in YAML

—--
sites:
    - name: san-jose # This is a comment. Begin block 1.
        hosts:
            - hostname: r1
              host: 192.168.1.1
              os: cisco_ios
            - hostname: sw1
              host: 192.168.1.5
              os: cisco_ios
            - hostname: sw2
              host: 192.168.1.6
              os: junos

    - name: san-diego # This is a comment. Begin block 2.
        hosts:
            - hostname: r2
              host: 192.168.2.1
              os: cisco_ios
            - hostname: sw1
              host: 192.168.2.5
              os: cisco_ios

Have we created a list or a dictionary? List items which is a dictionary.

Here’s a sample YAML script used to configure a VLAN on a Cisco switch.

---
- hosts: cisco
  connection: network_cli
  tasks:
    - nxos_vlan:
      vlan_id: 10
      name: STORAGE