Understanding Loops in Ansible

Understanding Loops in Ansible#

Loops in Ansible are used to repeat tasks multiple times. Ansible provides several keywords for looping, including loop, with_<lookup>, and until. These enable you to execute a task iteratively with different parameters, simplifying repetitive configurations.

Example: Configuring NTP Servers on a Router#

Let’s consider an Ansible playbook that configures NTP (Network Time Protocol) on a router:

---
- name: "Configure NTP"
  hosts: router
  gather_facts: false

  tasks:
    - name: "Configure NTP on Router"
      vars:
        ntp_server:
          - "1.1.1.1"
          - "2.2.2.2"
          - "3.3.3.3"
          - "4.4.4.4"
      cisco.ios.ios_ntp_global:
        config:
          peers:
            - peer: "{{ item }}"
              version: 2
      loop: "{{ ntp_server }}"

Playbook Explanation#

  • Play Definition: The play is named “Configure NTP” and targets the router host group. The directive gather_facts: false disables the default fact-gathering behavior.

  • Task Definition: A single task is defined within the play. This task uses the vars keyword to define a list of NTP servers.

  • Task Details:

    • Task Name: “Configure NTP on Router”

    • Variable Definition: The ntp_server variable is defined as a list of four IP addresses.

    • Module: The cisco.ios.ios_ntp_global module configures NTP settings on Cisco IOS devices.

    • Configuration: The config parameter specifies the NTP peers to be configured, using a loop to iterate over the ntp_server list.

    • Loop: The loop keyword iterates over the ntp_server list, configuring each NTP server in turn.

Understanding item in the Loop#

In Ansible loops, item represents the current element in the list being processed. As the loop iterates over the ntp_server list, item holds the value of each NTP server IP address one by one.

For example:

  • During the first iteration, item is 1.1.1.1.

  • During the second iteration, item is 2.2.2.2.

  • And so on for 3.3.3.3 and 4.4.4.4.

This allows the task to apply the configuration for each NTP server individually.

Running the Playbook#

To execute the playbook, you would use the following command:

ansible-playbook ntp_config.yaml

Playbook Output#

The playbook output might resemble this:

PLAY [Configure NTP] *******************************************************************

TASK [Configure NTP on Router] *********************************************************
changed: [172.16.10.14] => (item=1.1.1.1)
changed: [172.16.10.14] => (item=2.2.2.2)
changed: [172.16.10.14] => (item=3.3.3.3)
changed: [172.16.10.14] => (item=4.4.4.4)

PLAY RECAP *****************************************************************************
172.16.10.14               : ok=1    changed=1    unreachable=0    failed=0    skipped=0

Analysis of Playbook Output#

  • Play Execution: The play named “Configure NTP” begins execution.

  • Task Execution: The task “Configure NTP on Router” executes for each item in the ntp_server list.

    • For each IP address in the ntp_server list, a configuration change is applied to the router.

    • The output confirms successful configuration changes for each NTP server.

  • Play Recap: The play recap summarizes the execution:

    • ok=1: One task executed successfully.

    • changed=1: The task made configuration changes.

    • unreachable=0: No hosts were unreachable.

    • failed=0: No tasks failed.

    • skipped=0: No tasks were skipped.

This example demonstrates the use of loops in Ansible to efficiently configure multiple items, simplifying the management of network devices.

Loop Through Device Information#

To process device information, we use the ios_facts module, which collects facts from a device running IOS.

Here’s how it works:

---
- name: "Playbook: Loop IP Addr"
  hosts: router
  gather_facts: false

  tasks:
    - name: "Get facts from device"
      cisco.ios.ios_facts:
        gather_subset: interfaces

    - name: "Print device information"
      ansible.builtin.debug:
        msg: "{{ ansible_facts['net_hostname'] }} has IP Addr {{ item }}"
      loop: "{{ ansible_facts['net_all_ipv4_addresses'] }}"

Playbook Explanation#

This playbook, named “Loop IP Addr,” is designed to run on a host group named “router.” The gather_facts option is set to false because we manually gather the necessary facts using the ios_facts module.

Tasks#

  1. Get facts from device

    • The cisco.ios.ios_facts module is used to gather information from the device, specifically focusing on the interfaces subset.

  2. Print device information

    • The ansible.builtin.debug module is used to print the device’s hostname and its IP addresses.

    • The loop directive iterates over the list of IP addresses stored in ansible_facts['net_all_ipv4_addresses'].

Understanding the Loop#

In the second task, the loop takes each IP address from the list ansible_facts['net_all_ipv4_addresses'] and prints a message. The message includes the device’s hostname and the current IP address ({{ item }}) from the loop. This way, each IP address of the device is printed along with the device’s hostname.

PLAY [Playbook: Loop IP Addr] **********************************************************

TASK [Get facts from device] ***********************************************************
ok: [172.16.10.14]

TASK [Print device information] ********************************************************
ok: [172.16.10.14] => (item=9.9.9.10) => {
    "msg": "R1 has IP Addr 9.9.9.10"
}
ok: [172.16.10.14] => (item=172.16.10.14) => {
    "msg": "R1 has IP Addr 172.16.10.14"
}
ok: [172.16.10.14] => (item=192.168.71.139) => {
    "msg": "R1 has IP Addr 192.168.71.139"
}
ok: [172.16.10.14] => (item=1.1.1.1) => {
    "msg": "R1 has IP Addr 1.1.1.1"
}

PLAY RECAP *****************************************************************************
172.16.10.14               : ok=2    changed=0    unreachable=0    failed=0    skipped=0

As per documentation, we can modify the playbook:

---
- name: "Playbook: Loop IP Addr"
  hosts: router
  gather_facts: false

  tasks:
    - name: "Get facts from device"
      cisco.ios.ios_facts:
        gather_subset: interfaces

    - name: "Print device information"
      ansible.builtin.debug:
        msg: "{{ ansible_net_hostname }} has IP Addr {{ item }}"
      loop: "{{ ansible_net_all_ipv4_addresses }}"

This playbook demonstrates how to gather device information using the ios_facts module and print each IP address associated with a device. By utilizing the debug module and looping through the IP addresses, the playbook provides a clear and concise way to display critical network information for devices running IOS.

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