Jinja2 Extensions in netlab Configuration Templates

Starting with release 26.01, netlab generates all configuration files internally using the standard Jinja2 Python library. This approach makes netlab more stable and independent of Ansible whims (and associated headaches), but also makes most Ansible-specific Jinja2 filters unavailable in netlab configuration templates.

Additional Jinja2 Filters

However, to make it easier to write configuration templates for network devices, netlab includes an independent implementation of these Ansible filters:

• ipaddr, ipv4, ipv6, and hwaddr, either as simple filter names or with ansible.utils or ansible.netcommon prefix

• combine, difference, flatten, to_yaml, and to_nice_yaml, either as simple filter names or with ansible.builtin prefix

Note

The ‌to_yaml and ‌to_nice_yaml filters return identical results – nicely formatted YAML strings. The extra arguments used with these filters are ignored

Please open a GitHub issue if you use another Ansible-specific Jinja2 filter in your templates.

The ipaddr, ipv4, and ipv6 filters recognize these arguments:

argument	meaning
no argument	Select IP addresses from input string or list
integer or number as string	Return N-th IP address in the input subnet as a CIDR prefix
address	Return the input value as an IP address (without the prefix)
host	Generate an IP prefix from the input address or prefix
prefix	Return the prefix length of the input IP prefix
subnet	Return the input value as a CIDR prefix
other arguments	Return the corresponding netaddr.IPNetwork attribute

Warning

The ‌ipaddr filter does not accept ‌bool input values. The input value must be a string or a list (if you use ‌ipaddr with no extra arguments).

You don’t have to check the validity of IP addresses for netlab attributes; netlab does that during the input data validation phase. To differentiate between unnumbered interfaces (where the ‌ipv4 or ‌ipv6 interface attribute is set to ‌True) and numbered interfaces, use the intf.ipv4 is string test.

Undefined Variables

netlab emulates the Ansible implementation of undefined variables that allows you to test the presence of an element in a nested dictionary without checking the presence of the parent variable.

For example:

• You can test whether loopback.ipv4 is defined even on nodes that have no loopback interface. Standard Jinja2 would require you to use loopback is defined and loopback.ipv4 is defined.

• You can use the default filter on elements in nested dictionaries. For example, loopback.ipv4|default('10.0.0.1') will return 10.0.0.1 if the loopback interface has no IPv4 address, or if the device has no loopback interface.

Warning

Do not use undefined values with the ‌ipaddr filter. If you think you have to check the input data against the ‌ipaddr filter, use some_input is defined and some_input|ipaddr. Alternatively, use some_input|default('')|ipaddr

For consistency with built-in configuration templates, use is defined instead of "key" in dictionary. For example, use loopback.ipv4 is defined instead of "ipv4" in loopback. Unless you can safely assume that the loopback is a dictionary, a safer test using the “a in b” paradigm is loopback is defined and "ipv4" in loopback.