Topology Defaults

netlab has dozens of system defaults that specify:

Changing System Defaults

All system defaults specified in the global topology-defaults.yml file (shipped with netlab package) could be overwritten with the defaults element within the lab topology, or from user- or system- defaults files:

  • topology-defaults.yml file residing in the same directory as the topology file or in the user’s home directory.

  • .netlab.yml file residing in user’s home directory

  • /etc/netlab/defaults.yml file

The topology defaults value is most commonly used to set default device type; you could also use it to set any other default parameter. For example, the following topology file builds a network of Cisco IOSv devices using a different value for the default IS-IS area:

  device: iosv
    area: 49.0002


When augmenting default settings, netlab uses a deep dictionary merge, allowing you to overwrite a single setting deep in the hierarchy without affecting any other related settings.

For example, it’s possible to replace the default Vagrant box name for a network device type without changing any other device parameter[1]:

defaults.device: eos
defaults.devices.eos.image: arista/vEOS-lab-4.21.14M

Note: If you want to run your topology on multiple virtualization provides, you can set different device image name for every virtualization provider:

defaults.device: eos
defaults.devices.eos.libvirt.image: arista/vEOS-lab-4.21.14M
defaults.devices.eos.clab.image: cEOS:latest

Deep Merging

netlab uses Python Box package to perform recursive merge of configuration dictionaries:

  • Dictionary keys not present in target dictionary are added from the defaults dictionary;

  • Whenever a key in the target dictionary is itself a dictionary, and the same key is present in the defaults dictionary, the merge process recurses, resulting in a recursive merge of child dictionaries.

  • Lists and scalar values are not merged.

User Default Settings

netlab reads system defaults from the system topology-defaults.yml and augments them with:

  • topology-defaults.yml file in the lab directory

  • .netlab.yml file in the user home directory.

  • System-wide /etc/defaults/netlab.yml file

The defaults setting in the user defaults files are identical to the ones you’d use in the network topology file but without the defaults. prefix.

For example, to make sure all labs use Arista EOS as the default device type using a specific cEOS or vEOS image, create the following .netlab.yml file in your home directory:

device: eos
  clab.image: ceos:4.27.2F
  libvirt.image: arista/veos:4.28.3M

Alternate Defaults Files Locations (Advanced)

By default, netlab tries to find:

  • User default files in ./topology-defaults.yml, ~/.netlab.yml and ~/topology-defaults.yml

  • System defaults in /etc/netlab/defaults.ymland package:topology-defaults.yml

You can change the locations of user- or system defaults with the defaults.sources parameters specified in the lab topology file[2]:

  • defaults.sources.extra adds files to the list of user default files. You can use this parameter to add extra defaults to larger projects with a hierarchical directory structure.

  • defaults.sources.list (if present) specifies the complete list of default file locations that cannot be changed with other parameters

  • defaults.sources.user changes the list of potential user default files.

  • defaults.sources.system changes the list of potential system default files.


If you feel the need to change defaults.sources.system setting make sure package:topology-defaults.yml is the last element in the list or you’ll face an interesting troubleshooting exercise.

For example, the project-wide defaults file in the BGP Hands-On Labs project specifies the device type you want to use in the labs. Individual lab topologies (example) are stored in subdirectories and use defaults.sources.extra parameter to add project-wide defaults to the lab topology, for example:

# Configure a BGP session with an external BGP speaker

defaults.sources.extra: [ ../defaults.yml ]

    device: cumulus
    module: [ bgp ] 65100

- rtr-x1