Topology Defaults

netlab has dozens of system defaults that specify:

Changing Defaults

The system defaults are specified in the global topology-defaults.yml file (shipped with the netlab package). These defaults are imported into the lab topology under the defaults top-level attribute. They could be overwritten with the defaults element within the lab topology or from user- or system defaults files.

Changing Defaults in Lab Topology

To change a default setting for a single lab topology, set defaults.$something attribute to the desired value in the lab topology file.

For example, if you want to change the current lab’s default device (defaults.device) to Arista EOS, add defaults.device: eos to the lab topology file or use the expanded format:

  device: eos


If you’re a novice user, tweak the defaults with the lab topology settings and migrate them to the user defaults file when you get the desired results.

Changing Defaults in User Defaults Files

Create a YAML file specifying user defaults if you want the same defaults changed for all your lab topologies. netlab tries to find user defaults in the following files (but see also Alternate Defaults Files Locations):

  • topology-defaults.yml file in the lab directory

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

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

The default settings in the user defaults files are identical to the ones you’d use in the network topology file but without the defaults. prefix[1].

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:4.31.2F
  libvirt.image: arista/veos:4.28.3M

During the topology gathering process, netlab reads the ~/.netlab.yml file, converts it into a dictionary, and merges it with the defaults topology dictionary, resulting in the defaults.device entry in the lab topology data structure set to eos.


If you don’t know what to change, explore the system defaults with the ‌netlab show defaults command. See also Debugging User Default Files.

Example: Changing Device Type and Device Image

The topology defaults value is most commonly used to set the 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 defaults hierarchy without affecting 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[2]:

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

Change the provider-specific device images if you want to run your topology on multiple virtualization providers. This is how you can set different device image names for Arista EOS virtual machines and containers in the lab topology:

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

You can also change the default image names in the user defaults file:

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

Changing Defaults with Environment Variables

You can change netlab defaults with environment variables starting with netlab_ or NETLAB_. These environment variables are processed as follows:

  • Convert the variable name to lowercase.

  • Remove the netlab_ prefix.

  • Replace underscores with dots. Replace double underscores with underscores.

  • Set the corresponding value in the topology defaults.

For example, the NETLAB_DEVICE variable sets the defaults.device parameter and the NETLAB_BGP_AS variable sets the parameter.

You can use the environment variables instead of the --device, --provider, or --set arguments of the netlab up command. For example, the following command sequence starts a lab topology using Arista EOS containers:

$ export NETLAB_DEVICE=eos
$ export NETLAB_PROVIDER=clab
$ netlab up

Some netlab defaults have an underscore in their names. To set those parameters with environment variables, use a double underscore. For example, to set the libvirt batch_size parameter, use:


Environment variables have higher precedence than the system- and user defaults but cannot override the netlab up CLI arguments. For example, this is the order of precedence (from highest to lowest) used to select the virtualization provider:

  • The --provider CLI argument

  • provider attribute specified in lab topology

  • NETLAB_PROVIDER environment variable

  • defaults.provider attribute in the lab topology

  • provider attribute in user defaults

  • provider attribute is not specified in the system defaults but would have been the value of last resort.

Debugging User Default Files

You can debug the processing of the default files with the --debug defaults option of the netlab create command. You can also debug default settings without creating any output with the netlab create --output none --debug defaults command.

Use the netlab show defaults command to investigate how the system defaults were augmented with the user defaults. For example, execute netlab show defaults gateway to inspect the settings of the gateway module or netlab show defaults outputs.d2 to examine the settings of the D2 graphing output module.

You can also use the netlab inspect defaults… command after you create the lab configuration files (and the snapshot file) with the netlab create or netlab up command. For example, to inspect the Arista EOS device settings, use netlab inspect defaults.devices.eos.

Finally, if you want to see how the lab topology defaults interact with user- and system defaults without creating any output files, use the netlab create –output yaml:defaults… command to process the lab topology and print the collected default values. For example, execute netlab create --output yaml:defaults.addressing to display the default address pools your lab topology would use.

Advanced Topics

Deep Merging

netlab uses the Python Box package to perform a recursive merge of lab topology with user- and system defaults:

  • Dictionary keys not present in the 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.

Alternate Defaults Files Locations

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.yml and 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[3]:

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

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

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

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


If you need to change the defaults.sources.system or default.sources.list settings, make sure package:topology-defaults.yml is the last element in the list.

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