TL&DR: If you’re using netlab to build labs for your personal use, you can skip this one, but if you plan to use it to create training labs (like my BGP labs project), you might want to keep reading.

Like any complex enough tool, netlab eventually had to deal with inconsistent version-specific functionality and configuration syntax (OK, topology attributes). I stumbled upon this challenge when I wanted to make labs that use two types of configurable devices.

When customizing the BGP labs, you can specify the default device type. It can be any of the 20 or so platforms for which we implemented baseline BGP functionality. The labs also contain devices (external routers) that must be preconfigured, and I decided to use Cumulus Linux on them. Not a problem; I created a group, put all external routers into that group, and specified the device type for the group. This is the relevant part of the original lab topology for the ‌Configure Multiple EBGP Sessions lab exercise:

groups:
  external:
    members: [ x1, x2 ]
    module: [ bgp ]
    device: cumulus

A few months later, we implemented most of the additional functionality needed on the external routers in bgp.session and bgp.policy plugins, making it possible to use devices other than Cumulus Linux for external routers (more details). Still, you had to change the lab topology files if you wanted to use those devices as external routers. Time for another nerd knob: default group settings introduced in netlab release 1.6.4.

With the default group settings, you can specify the desired device type for the external group in lab defaults, for example:

device: eos             # Change to your preferred network device
provider: clab          # Change to virtualbox or libvirt if needed

groups:
  external:
    device: cumulus     # Change to your preferred external router. Needs 1.6.4 to work

However, the lab topology settings override the default ones, so I was in a Catch-22 situation:

• If I set the device type for the external group in lab topology, then the default group settings don’t apply.
• If I don’t set the external group device type in lab topology, the lab won’t work as expected if the user running it uses a netlab release older than 1.6.4.

How about yet another nerd knob: version-specific topology files. If a directory contains topology.yml (default filename for the lab topology file) and topology.1.6.4.yml, netlab versions lower than 1.6.4 use topology.yml while newer versions use topology.1.6.4.yml. You can use the same trick multiple times and have (for example) yet another topology file named topology.1.7.3.yml that will be used only if you’re running netlab release 1.7.3 or later.

Tiny differences due to default group settings are just one reason I’m using this approach in BGP labs, for example, in the Multiple EBGP Sessions lab. I’m also using version-specific topology files whenever I can benefit from the new BGP plugin functionality, for example, in the Filter Transit Routes lab, where I use the bgp.policy plugin to change the BGP local preference of the customer routes.