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First Step: Configuration Templates

Articles » Yet Another Network Automation Journey » First Step: Configuration Templates


This article was written by Anne Baretta, network architect at Falco Networks.

Creating configuration templates with Jinja2 is an excellent initial use-case for Ansible. One very beneficial aspect of any network automation journey is that it encourages a systematic and progressively organized approach to network engineering. Before long, you will have weeded out all those pesky exceptions in your network, as they hinder any automation effort you would like to subject them to, and conversely uniformity brings great rewards.

You should always start with the Ansible inventory, which should be a structured reflection of the makeup of your install base, and continue to be consistent when defining configuration templates. Use templates to define the configuration standard with global management parameters, access-lists, policies..., and get rid manually hacked together configs with omissions and deviations from the standard. At the very least, you can start by ensuring no new devices will be deployed with configuration deviating from the defined standard.

Tip for Cisco IOS templates: include an EEM applet in the template to have the device generate a key and enable ssh upon booting, see the appendix.

With the inventory set up, the next step is to use Ansible to interact with devices directly. A useful example is preparing network devices for firmware upgrades. I wrote a playbook that prepared all devices for upgrading, leaving the actual software reload as the only manual step required to activate the new image. However, after creating an elaborate playbook doing all kinds of checks, I realized a better approach for upgrading Cisco IOS devices is grabbing the tar file and using the archive download-sw command. With this command, you can do everything, including checks on platform compatibility, feature set, and memory. It can also set the boot statement and reload the device (if you want to do that)... all with a single command (refer to the appendix for the available options for the download-sw command).

The playbook can then be as simple as this:

- name: Upgrade IOS image if not compliant
  block:
  - name: Run download-sw for IOS image
    ios_command:
      commands:
        - "archive download-sw /imageonly tftp://{{TFTPHOST}}/{{fw_tar}}"
      wait_for: |
        result[0] contains All software images installed.
  when: |
    ansible_net_version != compliant_ios_version
    and not ansible_net_stacked_models[1:]

The ansible_net_stacked_models[1:] expression is true when there is more than one switch present in the ansible_net_stacked_models array; in other words, when the device is a stack. Note that waiting for the confirmation that the software image is installed can take a while. To be able to have Ansible catch the confirmation in the playbook, increase the Ansible command_timeout to a sufficiently long time. I found that while most switches complete the procedure within five or six minutes, sometimes it takes much longer for the same device. Regardless of whether Ansible decides to time out the operation, the switch will continue to complete the download-sw command in the background, and so far, each time the switch eventually got through it. An assert task which runs after a while to doublecheck may be in order. YMMV.

The fw_tar variable can be the result of a database or CSV lookup for the compliant_ios_version. In my case, the compliant_ios_version is queried from the database, which brings me to the next step in the network automation journey: collecting Ansible outputs and manipulating them in a database.


This article is an account of how to get started with network automation, and some common use cases. You can reach Anne via LinkedIn.