My initial “configuring Arista cEOS with Netmiko or Ansible” test was a bit biased. netlab generates the final configuration snippets in both cases (due to Ansible FUBARs we had to deal with), but then Netmiko just sends the configuration commands to the switch (after entering configuration mode), while the Ansible eos_config module executes show running, identifies the difference, and sends only the changes to the device.

While that use case is highly relevant (after all, we’re configuring network devices ;) I really should have done an apples-to-apples comparison, and FRRouting virtual machines are the ideal playground.

The FRRouting virtual machines are always configured with bash or vtysh scripts. The script is first sent to the device, then executed on it. This is the relevant Ansible task list¹:

- template:
    src: "{{ config_template }}"
    dest: /tmp/config.sh

- set_fact: deployed_config={{ lookup('template',config_template) }}

- name: "run /tmp/config.sh to deploy {{netsim_action}} config from {{ config_template }}"
  command: bash /tmp/config.sh
  when: not ansible_check_mode and ("#!/bin/bash" in deployed_config or "#!/bin/sh" in deployed_config)
  become: true

- name: "run vtysh to import {{netsim_action}} config from {{ config_template }}"
  command: vtysh -f /tmp/config.sh
  when: not ansible_check_mode and not ("#!/bin/bash" in deployed_config or "#!/bin/sh" in deployed_config)
  become: true

The equivalent Netmiko code is (almost) trivial. It uses the scp library to push the script to the device, and then executes the script over an already-opened SSH session using a crazy workaround to detect script errors:

    cmd_marker = "__NETMIKO_RC:" # Use a marker to get back the return code
    cmd = f'chmod a+x {cfg_file}; {cfg_file} 2>&1; rc=$?; echo {cmd_marker}$rc'

Like in the previous test, I used an EVPN lab and measured the startup (Vagrant starting Linux VMs) and configuration (Netmiko or Ansible doing their job) times.

Here are the results:

Operation	netlab CPU time2	Elapsed time
Starting the lab	4.5 s	25.7 s
Configuration with Netmiko	0.5 s	2.2 s
Configuration with Ansible	3.0 s	11.4 s

It takes almost half a minute to start the Linux virtual machines³, but then Netmiko delivers blazingly fast performance, while Ansible remains Ansible. Admittedly, the “push template and execute a script” operations are much faster than the equivalent eos_config operations (which took over half a minute), but it’s still five times slower than the alternative.