A few days ago, I described how you can use the new config.inline functionality to apply additional configuration commands to individual devices in a netlab-powered lab.

However, sometimes you have to apply the same set of commands to several devices. Although you could use device groups to do that, netlab release 25.09 offers a much better mechanism: you can embed custom configuration templates in the lab topology file.

The files plugin (used to implement the inline config functionality) also recognizes the configlets dictionary that can be used to create custom configuration templates when starting the lab.

For example, if you want to have ifup and ifdown templates you could use with netlab config command to enable or shut down an interface, you could add these lines to the lab topology file:

configlets:
  ifup: |
    #!/bin/bash
    ip link set eth1 up
  ifdown: |
    #!/bin/bash
    ip link set eth1 down

Based on the above definition, the files plugin creates ifup.j2 and ifdown.j2 files in the lab directory whenever you execute the netlab up or netlab create commands¹.

These commands would work on Linux, FRRouting, BIRD, or dnsmasq nodes (because they all use pure Linux with no custom CLI), but what if you’d like to have multi-vendor templates? No worries, the value of a configlets dictionary entry could be another dictionary, for example:

configlets:
  ifup:
    eos: |
      interface Ethernet1
        no shutdown
    frr: |
      #!/bin/bash
      ip link set eth1 up

The above dictionary would result in two files in the ifup subdirectory of the lab directory: ifup/eos.j2 and ifup/frr.j2.

This nicely solves the shut down an interface on Arista EOS or FRR challenge, but now we can’t use the same configuration template on Linux or BIRD devices unless we add more entries to the configlets.ifup dictionary. Even worse, those entries would be identical to the frr entry, and you know I hate data duplication.

Unfortunately, there’s not much we can do today. While I implemented a special base entry that would allow you to implement the use this other template for all other devices functionality, it doesn’t work well in this particular scenario (but does work for the more specific templates described in the documentation). The fix is coming in release 25.11 (at which point I’ll also update this blog post)