netsim-tools have dozens of system defaults that specify:
Default addressing pools
Default virtualization provider
Virtualization provider defaults (configuration file name, port mappings, provider-specific device defaults…)
Device parameters (interface names, image names…)
All system defaults, specified in global topology-defaults.yml file (shipped within netsim package directory), could be overwritten with topology- or user defaults:
In defaults element of the topology file
In topology-defaults.yml file residing in the same directory as the topology file or in the 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:
--- defaults: device: iosv isis: area: 49.0002 ...
When augmenting default settings, netsim-tools use 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:
--- defaults: device: eos devices: eos: image: libvirt: arista/vEOS-lab-4.21.14M
netsim-tools library 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.
netsim-tools read system defaults from the system topology-defaults.yml and augment them with the settings from topology-defaults.yml file in the lab directory or user home directory.
For example, to make sure all labs use Arista EOS as the default device type, create the following topology-defaults.yml file in your home directory:
--- device: eos