Template inheritance

Template inheritance is a very powerful functionality that avoids repetition of the same in different templates.

When using inheritance, there are:

  • base template - template that describes template skeleton.
    • this template may contain any ordinary expressions or text. In addition, special blocks are defined in this template.
  • child template - template that extends base template by filling in specified blocks.
    • child templates can overwrite or supplement blocks defined in base template.

Example of base template templates/base_router.txt:

!
{% block services %}
service timestamps debug datetime msec localtime show-timezone year
service timestamps log datetime msec localtime show-timezone year
service password-encryption
service sequence-numbers
{% endblock %}
!
no ip domain lookup
!
ip ssh version 2
!
{% block ospf %}
router ospf 1
 auto-cost reference-bandwidth 10000
{% endblock %}
!
{% block bgp %}
{% endblock %}
!
{% block alias %}
{% endblock %}
!
line con 0
 logging synchronous
 history size 100
line vty 0 4
 logging synchronous
 history size 100
 transport input ssh
!

Note four blocks that are created in template:

{% block services %}
service timestamps debug datetime msec localtime show-timezone year
service timestamps log datetime msec localtime show-timezone year
service password-encryption
service sequence-numbers
{% endblock %}
!
{% block ospf %}
router ospf 1
 auto-cost reference-bandwidth 10000
{% endblock %}
!
{% block bgp %}
{% endblock %}
!
{% block alias %}
{% endblock %}

These are blanks for the corresponding configuration sections. A child template that uses this base template as a base can fill all or only some of the blocks.

Child template templates/hq_router.txt:

{% extends "base_router.txt" %}

{% block ospf %}
{{ super() }}
{% for networks in ospf %}
 network {{ networks.network }} area {{ networks.area }}
{% endfor %}
{% endblock %}

{% block alias %}
alias configure sh do sh
alias exec ospf sh run | s ^router ospf
alias exec bri show ip int bri | exc unass
alias exec id show int desc
alias exec top sh proc cpu sorted | excl 0.00%__0.00%__0.00%
alias exec c conf t
alias exec diff sh archive config differences nvram:startup-config system:running-config
alias exec desc sh int desc | ex down
{% endblock %}

The first line in template templates/hq_router.txt is very important:

{% extends "base_router.txt" %}

It is said that template hq_router.txt will be constructed on the basis of template base_router.txt.

Inside child template, everything happens inside blocks. Due to the blocks that have been defined in base template, child template can extend the parent template.

Note

Note that lines described in child template outside blocks are ignored.

There are four blocks in base template: services, ospf, bgp, alias. In child template only two of them are filled: ospf and alias. That’s the convenience of inheritance. You don’t have to fill all blocks in every child template.

In this way ospf and alias blocks are used differently. In base template, ospf block already has part of configuration:

{% block ospf %}
router ospf 1
 auto-cost reference-bandwidth 10000
{% endblock %}

Therefore, child template has a choice: use this configuration and supplement it or completely rewrite everything in child template.

In this case the configuration is supplemented. That is why in child template templates/hq_router.txt the ospf block starts with expression {{ super() }}:

{% block ospf %}
{{ super() }}
 {% for networks in ospf %}
 network {{ networks.network }} area {{ networks.area }}
 {% endfor %}
{% endblock %}

{{ super() }} transfers content of this block from parent template to child template. Because of this, lines from parent are moved to child template.

Note

Expression super doesn’t have to be at the beginning of the block. It could be anywhere in the block. Content of base template are moved to where super expression is located.

alias block simply describes the alias. And even if there were some settings in parent template, they would be substituted by content of child template.

Let’s recap the rules for working with blocks. If block is created in parent template:

  • no content - in child template you can fill this block or ignore it. If block is filled, it will contain only what was written in child template (example - alias block)
  • with content - in child template you can perform such actions:
    • ignore block - in this case, child template will get content from parent template (example - services block)
    • rewrite block - then child template will contain only what it has
    • move content of the block from parent template and supplement it - then child template will contain both the content of the block from parent template and the content from child template. To pass content from parent template the expression {{ super() }} is used (example - ospf block)

Data file for template configuration generation (data_files/hq_router.yml):

ospf:
  - network: 10.0.1.0 0.0.0.255
    area: 0
  - network: 10.0.2.0 0.0.0.255
    area: 2
  - network: 10.1.1.0 0.0.0.255
    area: 0

The result will be:

$ python cfg_gen.py templates/hq_router.txt data_files/hq_router.yml
!
service timestamps debug datetime msec localtime show-timezone year
service timestamps log datetime msec localtime show-timezone year
service password-encryption
service sequence-numbers
!
no ip domain lookup
!
ip ssh version 2
!
router ospf 1
 auto-cost reference-bandwidth 10000

 network 10.0.1.0 0.0.0.255 area 0
 network 10.0.2.0 0.0.0.255 area 2
 network 10.1.1.0 0.0.0.255 area 0
!
!
alias configure sh do sh
alias exec ospf sh run | s ^router ospf
alias exec bri show ip int bri | exc unass
alias exec id show int desc
alias exec top sh proc cpu sorted | excl 0.00%__0.00%__0.00%
alias exec c conf t
alias exec diff sh archive config differences nvram:startup-config system:running-config
alias exec desc sh int desc | ex down
!
line con 0
 logging synchronous
 history size 100
line vty 0 4
 logging synchronous
 history size 100
 transport input ssh
!

Note that in ospf block there are commands from base template and commands from child template.