A previous blog post described how you can use the netlab report functionality to generate addressing, wiring, BGP, and OSPF reports from a running lab. But what could you do if you need a report that doesn’t exist yet? It’s straightforward to define one (what else did you expect?).

Let’s create the report I used in the EVPN Hub-and-Spoke Layer-3 VPN blog post to create the VRF table.

We need two bits of information to start our process:

• What should be the filename of the report definition?
• Where do we have to store it?

netlab report provides both answers when you invoke it with an unknown report name:

$ netlab report vrf.md
[ERRORS]  Errors found in topology.yml
[VALUE]   report: Cannot find "vrf.md.j2" in any of the report directories
[HINT]    A report can be specified in a file with .j2 suffix within 'reports' subdirectory in package-,
          system-, user- or current directory. You can also specify a report in a defaults.outputs.report
          setting.

Next, we must figure out where to get the information we want to display in the report. netlab report takes the information from the transformed data model (from the netlab.snapshot.yml file). That file is created when you start the lab with the netlab up command; you can also create it with the netlab create command.

You can inspect the transformed data with a text editor (it’s a YAML file), with a tool like yq, or with the netlab inspect command. I know the global VRF data is in the vrfs dictionary, so it was easy to display it:

$ netlab inspect vrfs
hub_egress:
  evpn:
    evi: 4
    transit_vni: 200003
  export:
  - '65000:4'
  id: 4
  import:
  - '65000:4'
  rd: '65000:4'
hub_ingress:
  evpn:
    evi: 3
    transit_vni: 200002
  export:
  - '65000:3'
  id: 3
  import:
  - '65000:3'
  rd: '65000:3'
s_1:
  evpn:
    evi: 1
    transit_vni: 200000
  export:
  - '65000:4'
  id: 1
  import:
  - '65000:3'
  rd: '65000:1'
s_2:
  evpn:
    evi: 2
    transit_vni: 200001
  export:
  - '65000:4'
  id: 2
  import:
  - '65000:3'
  rd: '65000:2'

Now we have all the information we need. Our report has to iterate over the vrfs dictionary and display relevant bits from it. Here’s the first approximation:

{# description: VRF parameters #}
| VRF | RD | Export RT | Import RT | EVPN VNI |
|-----|---:|-----------|-----------|---------:|
{% for vname,vdata in vrfs.items() %}
| {{ vname }} | {{ vdata.rd }} | {{ vdata.export }} | {{ vdata.import }} | {{ vdata.evpn.transit_vni }} |
{% endfor %}

That’s good enough for what I needed, but we can do better. For example, the import and export route targets are displayed as Python lists:

$ netlab report vrf.md
| VRF | RD | Export RT | Import RT | EVPN VNI |
|-----|---:|-----------|-----------|---------:|
| hub_egress | 65000:4 | ['65000:4'] | ['65000:4'] | 200003 |
| hub_ingress | 65000:3 | ['65000:3'] | ['65000:3'] | 200002 |
| s_1 | 65000:1 | ['65000:4'] | ['65000:3'] | 200000 |
| s_2 | 65000:2 | ['65000:4'] | ['65000:3'] | 200001 |

Also, vdata.evpn.transit_vni (or even vdata.evpn) might not be defined.

To fix the Python lists, we have to pipe the import and export route targets through a join filter. While doing that, we can also supply the default value in case the attribute is missing:

vdata.import|default([])|join('<br>')

Dealing with vdata.evpn.transit_vni is a bit harder as we’re dealing with two levels of dictionaries, so we have to use something like this:

vdata.evpn.transit_vni|default('') if vdata.evpn is defined else ''

How do you put convoluted expressions like the ones above into a single like (to generate Markdown markup) without making the template look like a single-line Perl contortion? Jinja2 allows you to start a new line immediately after the {{ sequence, so you can put each expression into a separate line, resulting in the final report:

| VRF | RD | Export RT | Import RT | EVPN VNI |
|-----|---:|-----------|-----------|---------:|
{% for vname,vdata in vrfs.items() %}
| {{ vname }} | {{ 
     vdata.rd|default('') }} | {{ 
     vdata.export|default([])|join('<br>') }} | {{
     vdata.import|default([])|join('<br>') }} | {{
     vdata.evpn.transit_vni|default('') if vdata.evpn is defined else '' }} |
{% endfor %}

Have you created your own custom reports in the past? I’d love to hear about them. Leave a comment or email me (or, even better, submit a pull request).