Template an OSPF fabric from a vars file

Scenario

Three routers form an OSPF fabric: r1 - r2 - r3. Hand-writing each config doesn't scale. Your job is to write one Jinja2 template that renders every router's config from a shared vars file. The grader renders your template once per router (passing that router's data as router) and checks the resulting network actually converges.

The vars (read-only)

Each router is rendered with a router variable shaped like this:

routers:
  - name: r1
    loopback: 1.1.1.1
    interfaces:
      - { name: eth1, ip: 10.0.12.1, len: 30 }
    networks: [ "1.1.1.1/32", "10.0.12.0/30" ]
  - name: r2
    loopback: 2.2.2.2
    interfaces:
      - { name: eth1, ip: 10.0.12.2, len: 30 }
      - { name: eth2, ip: 10.0.23.1, len: 30 }
    networks: [ "2.2.2.2/32", "10.0.12.0/30", "10.0.23.0/30" ]
  - name: r3
    loopback: 3.3.3.3
    interfaces:
      - { name: eth1, ip: 10.0.23.2, len: 30 }
    networks: [ "3.3.3.3/32", "10.0.23.0/30" ]

Your job

Edit template.j2 so that, for each router, it renders:

• the loopback and every interface address, and
• a router ospf block advertising all of that router's networks into area 0.

Supported template syntax

{{ expr }} interpolation (dotted paths like {{ router.name }}), {% for x in list %}…{% endfor %}, and {% if cond %}…{% endif %}.

What "done" looks like

A real template (not a pasted static config) that renders configs which bring up every OSPF adjacency, so r1 can reach 2.2.2.2 and 3.3.3.3.

Teaches: config templating, the core skill behind Ansible, Nornir, and every real network-automation pipeline.