Exercise 4 - Network Configuration with Jinja Templates

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Objective
Guide
Takeaways
Solution

Objective

Demonstration templating a network configuration and pushing it a device

Use and understand group variables to store the IP addresses we want.
Use the Jinja2 template lookup plugin
Demonstrate use of the network automation cli_config module

Guide

Step 1

This step will cover creating Ansible variables for use in an Ansible Playbook. This exercise will use the following IP address schema for loopbacks addresses on rtr1 and rtr2:

Device	Loopback100 IP
rtr1	192.168.100.1/32
rtr2	192.168.100.2/32

Variable information can be stored in host_vars and group_vars. For this exercise create a folder named group_vars:

[student1@ansible network-workshop]$ mkdir ~/network-workshop/group_vars

Now create a file in this directory name all.yml using your text editor of choice. Both vim and nano are installed on the control node.

[student1@ansible network-workshop]$ nano group_vars/all.yml

The interface and IP address information above must be stored as variables so that the Ansible playbook can use it. Start by making a simple YAML dictionary that stores the table listed above. Use a top level variable (e.g. nodes) so that a lookup can be performed based on the inventory_hostname:

nodes:
  rtr1:
    Loopback100: "192.168.100.1"
  rtr2:
    Loopback100: "192.168.100.2"

Copy the YAML dictionary we created above into the group_vars/all.yml file and save the file.

All devices are part of the group all by default. If we create a group named cisco only network devices belonging to that group would be able to access those variables.

Step 2

Create a new template file named template.j2:

[student1@ansible network-workshop]$ nano template.j2

Copy the following into the template.j2 file:

{% for interface,ip in nodes[inventory_hostname].items() %}
interface {{interface}}
  ip address {{ip}} 255.255.255.255
{% endfor %}

Save the file.

Step 3

This step will explain and elaborate on each part of the newly created template.j2 file.

{% for interface,ip in nodes[inventory_hostname].items() %}

Pieces of code in a Jinja template are escaped with {% and %}. The interface,ip breaks down the dictionary into a key named interface and a value named ip.

The nodes[inventory_hostname] does a dictionary lookup in the group_vars/all.yml file. The inventory_hostname is the name of the hostname as configured in Ansible’s inventory host file. When the playbook is executed against rtr1 inventory_hostname will be rtr1, when the playbook is executed against rtr2, the inventory_hostname will be rtr2 and so forth.

The inventory_hostname variable is considered a magic variable which is automatically provided.

The keyword items() returns a list of dictionaries. In this case the dictionary’s key is the interface name (e.g. Loopback100) and the value is an IP address (e.g. 192.168.100.1)

interface {{interface}}
  ip address {{ip}} 255.255.255.255

Variables are rendered with the curly braces like this: `` In this case the variable name key and value only exist in the context of the loop. Outside of the loop those two variables don’t exist. Each iteration will re-assign the variable name to new values based on what we have in our variables.

Finally:

{% endfor %}

In Jinja we need to specify the end of the loop.

Step 4

Create the Ansible Playbook config.yml:

[student1@ansible network-workshop]$ nano config.yml

Copy the following Ansible Playbook to the config.yml file:

---
- name: configure network devices
  hosts: rtr1,rtr2
  gather_facts: false
  tasks:
    - name: configure device with config
      cli_config:
        config: "{{ lookup('template', 'template.j2') }}"

This Ansible Playbook has one task named configure device with config
The cli_config module is vendor agnostic. This module will work identically for an Arista, Cisco and Juniper device. This module only works with the network_cli connection plugin.
The cli_config module only requires one parameter, in this case config which can point to a flat file, or in this case uses the lookup plugin. For a list of all available lookup plugins visit the documentation
Using the template lookup plugin requires two parameters, the plugin type template and the corresponding template name template.j2.

Step 5

Execute the Ansible Playbook:

[student1@ansible network-workshop]$ ansible-playbook config.yml

The output should look as follows.

[student1@ansible ~]$ ansible-playbook config.yml

PLAY [rtr1,rtr2] ********************************************************************************

TASK [configure device with config] ********************************************************************************
changed: [rtr1]
changed: [rtr2]

PLAY RECAP ********************************************************************************
rtr1                       : ok=1    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0
rtr2                       : ok=1    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

Step 6

Use the command show ip int br to verify the IP addresses have been confirmed on the network devices.

[student1@ansible network-workshop]$ ssh rtr1

rtr1#show ip int br | include Loopback100
Loopback100            192.168.100.1   YES manual up                    up

Takeaways

The Jinja2 template lookup plugin can allow us to template out a device configuration.
The *os_config (e.g. ios_config) and cli_config modules can source a jinja2 template file, and push directly to a device. If you want to just render a configuration locally on the control node, use the template module.
Variables are mostly commonly stored in group_vars and host_vars. This short example only used group_vars.

Solution

The finished Ansible Playbook is provided here for an answer key: config.yml.

The provided Ansible Jinja2 template is provided here: template.j2.

Complete

You have completed lab exercise 4

Click here to return to the Ansible Network Automation Workshop