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Jinja2 Template Support

日本語版 / Japanese | Config Subcommand

The config subcommand supports Jinja2 templates (.j2 files) to generate per-host set commands from a single template. This is useful when deploying configurations that vary by host — such as hostnames, NTP servers, or protocol settings based on device role.

Installation

Jinja2 is an optional dependency:

pip install junos-ops[template]

Note: Jinja2 is already included as a dependency of junos-eznc (PyEZ), so it may already be available without explicit installation.

Quick Start

1. Define variables in config.ini

Common variables go in [DEFAULT] and are inherited by all hosts. Host-specific values can override them.

[DEFAULT]
var_ntp_server = 192.0.2.1
var_syslog_host = 192.0.2.2

[rt1.example.jp]
tags = tokyo, core

[sw1.example.jp]
tags = tokyo, access

2. Create a template file

{# ntp.set.j2 — NTP and syslog configuration #}
set system host-name {{ hostname }}
set system ntp server {{ ntp_server }}
set system syslog host {{ syslog_host }} any warning
{% if facts.personality == 'SWITCH' %}
set protocols vstp
{% endif %}

3. Preview with dry-run

junos-ops config -f ntp.set.j2 --dry-run rt1.example.jp sw1.example.jp

For rt1.example.jp (router), the rendered output:

set system host-name rt1.example.jp
set system ntp server 192.0.2.1
set system syslog host 192.0.2.2 any warning

For sw1.example.jp (switch), the rendered output includes set protocols vstp:

set system host-name sw1.example.jp
set system ntp server 192.0.2.1
set system syslog host 192.0.2.2 any warning
set protocols vstp

4. Apply

junos-ops config -f ntp.set.j2 rt1.example.jp sw1.example.jp

Template Variables

Three sources of variables are available in templates:

Source Description Example
hostname Config section name {{ hostname }} → rt1.example.jp
var_* keys config.ini keys with var_ prefix (prefix stripped) var_ntp_server → {{ ntp_server }}
facts Device facts (dict) retrieved after NETCONF connection {{ facts.model }} → MX240

Variable precedence

var_ keys follow configparser's standard inheritance:

  1. Host section value (highest priority)
  2. [DEFAULT] section value (fallback)
[DEFAULT]
var_ntp_server = 192.0.2.1        # used by all hosts

[rt1.example.jp]
var_ntp_server = 192.0.2.99      # overrides DEFAULT for rt1 only

Device facts

The facts variable is a dict containing device information retrieved via NETCONF after connection. Common keys:

Key Example Description
facts.hostname rt1 Device hostname
facts.model MX240 Device model
facts.version 21.4R3-S5.4 JUNOS version
facts.personality MX, SWITCH, SRX_BRANCH Device type
facts.serialnumber ABC1234 Serial number

Both dot notation (facts.model) and bracket notation (facts['model']) work.

Template Patterns

Conditional configuration by device role

set system ntp server {{ ntp_server }}
{% if facts.personality == 'SWITCH' %}
set protocols vstp
set ethernet-switching-options storm-control interface all
{% elif facts.personality == 'SRX_BRANCH' %}
set security zones security-zone trust
{% endif %}

Loop: multiple NTP servers

Define a comma-separated list in config.ini:

[DEFAULT]
var_ntp_servers = 192.0.2.1,192.0.2.2,192.0.2.3

Use {% for %} to iterate:

{% for server in ntp_servers.split(',') %}
set system ntp server {{ server }}
{% endfor %}

Rendered output:

set system ntp server 192.0.2.1
set system ntp server 192.0.2.2
set system ntp server 192.0.2.3

Loop: SNMP community with multiple prefixes

[DEFAULT]
var_snmp_community = public
var_snmp_prefixes = 192.0.2.0/24,198.51.100.0/24,203.0.113.0/24
{% for prefix in snmp_prefixes.split(',') %}
set snmp community {{ snmp_community }} clients {{ prefix }}
{% endfor %}

Comments

Jinja2 comments ({# ... #}) and shell-style comments (# ...) are both supported. Both are removed from the final output.

{# This Jinja2 comment is removed during rendering #}
# This shell-style comment is removed after rendering
set system ntp server {{ ntp_server }}

Error Handling

Undefined variables

Templates use Jinja2's StrictUndefined mode. Any undefined variable causes an immediate error with a clear message — the template is never partially applied.

set system ntp server {{ undefined_var }}
jinja2.exceptions.UndefinedError: 'undefined_var' is not defined

Template syntax errors

Jinja2 syntax errors (e.g., missing {% endif %}) are caught before any configuration is sent to the device.

Jinja2 not installed

If Jinja2 is not available, an ImportError with installation instructions is raised:

ImportError: Jinja2 is required for template support. Install it with: pip install junos-ops[template]