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How do you use Jinja2 templates to generate network configs?

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Jinja2 separates data from config. The template holds the config text with placeholders; YAML or the source of truth holds the values per device. {{ var }} prints a value, {% for %} and {% if %} add loops and conditions, and filters such as upper, default or join transform values. The same template renders the same structure for every device, which is where consistency comes from.

Go deeper

Template

Output

hostname {{ hostname }}
{% for server in ntp_servers %}
ntp server {{ server }}
{% endfor %}
{% if snmp_location is defined %}
snmp-server location {{ snmp_location }}
{% endif %}

Data

YAML

hostname: ASW1
ntp_servers:
  - 10.0.0.5
  - 10.0.0.6
snmp_location: Building A floor 2

Render in Python

Python

import yaml
from jinja2 import Environment, FileSystemLoader, StrictUndefined

env = Environment(
    loader=FileSystemLoader("templates"),
    trim_blocks=True,
    lstrip_blocks=True,
    undefined=StrictUndefined,
)
template = env.get_template("base.j2")
with open("data/asw1.yml") as f:
    data = yaml.safe_load(f)
print(template.render(**data))

Output

hostname ASW1
ntp server 10.0.0.5
ntp server 10.0.0.6
snmp-server location Building A floor 2

Things that trip people

Whitespace. Without trim_blocks and lstrip_blocks, every {% %} line leaves a blank line or indentation. You can also trim per tag with {%- and -%}.
Undefined variables. The default prints an empty string, which can produce ntp server with no address. StrictUndefined makes it fail instead. Ansible fails on undefined variables by default.
Logic creep. Keep templates simple and push decisions into the data.

In Ansible, the ansible.builtin.template module renders a .j2 file, and modules such as ios_config accept rendered config through src.

They will ask next

How would you test a template?
Render it for a set of sample devices in CI and compare with known-good output files, and render with StrictUndefined so missing data fails the build. For extra safety, feed the rendered configs to Batfish or push them to a lab device.

Traps

Putting device-specific values inside the template instead of the data.
Forgetting whitespace control and shipping configs full of blank lines and stray spaces.

How did you do?