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Topology Defaults

netlab has dozens of system defaults that specify:

  • Default addressing pools

  • Default virtualization provider

  • Module defaults

  • Virtualization provider defaults (configuration file name, port mappings, provider-specific device defaults…)

  • Device parameters (interface names, image names…) and capabilities

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

  • With defaults element of the topology file

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

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

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 parameter1:

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.

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 – netlab merges user default settings with system default settings.

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


See Hierarchical Dictionaries in netlab YAML files for an in-depth explanation of why attributes with hierarchical names work in netlab