Deep dive on VLANS resource modules for network automation

February 19, 2020 by Sean Cavanaugh

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In October of 2019, as part of Red Hat Ansible Engine 2.9, the Ansible Network Automation team introduced the concept of resource modules.  These opinionated network modules make network automation easier and more consistent for those automating various network platforms in production.  The goal for resource modules was to avoid creating overly complex jinja2 templates for rendering network configuration. This blog post goes through the eos_vlans module for the Arista EOS network platform.  I walk through several examples and describe the use cases for each state parameter and how we envision these being used in real world scenarios.

Before starting let’s quickly explain the rationale behind naming of the network resource modules. Notice for resource modules that configure VLANs there is a singular form (eos_vlan, ios_vlan, junos_vlan, etc) and a plural form (eos_vlans, ios_vlans, junos_vlans).  The new resource modules are the plural form that we are covering today. We have deprecated the singular form. This was done so that those using existing network modules would not have their Ansible Playbooks stop working and have sufficient time to migrate to the new network automation modules.

 

VLAN Example

Let's start with an example of the eos_vlans resource module:

---
- name: add vlans
  hosts: arista
  gather_facts: false
  tasks:
    - name: add VLAN configuration
      eos_vlans:
        config:
          - name: desktops
            vlan_id: 20
          - name: servers
            vlan_id: 30
          - name: printers
            vlan_id: 40
          - name: DMZ
            vlan_id: 50

There is an implicit state parameter which defaults to merged (i.e. state: merged).  If we run this Ansible Playbook VLANs 20,30,40 and 50 will be merged into the running configuration of any device in the arista group.  The show vlan output on a new Arista switch will look like the following:

rtr2#show vlan
VLAN  Name                             Status    Ports
----- -------------------------------- --------- -------------------------------
1     default                          active
20    desktops                         active
30    servers                          active
40    printers                         active
50    DMZ                              active

while the running configuration will look like the following:

rtr2#show running-config | s vlan
vlan 20
   name desktops
!
vlan 30
   name servers
!
vlan 40
   name printers
!
vlan 50
   name DMZ

Now let’s make a change manually to the network configuration:

rtr2(config)#vlan 100
rtr2(config-vlan-100)#name artisanal_vlan
rtr2(config-vlan-100)#end
rtr2#wr
Copy completed successfully.

If I re-run the Ansible Playbook it returns with changed=0 because it only cares about the VLANs 20, 30, 40 and 50. It won’t remove VLAN 100 because we have the state parameter set to merged by default, so it only will merged the data model it knows about. It is just enforcing configuration policy of the VLANs I am sending.

Sean Blog 1

 

Using the 'state' parameter

What happens if I change the state parameter to replaced?  Just change the previous example to the following:

---
- name: add vlans
  hosts: arista
  gather_facts: false
  tasks:
    - name: add VLAN configuration
      eos_vlans:
        state: replaced
        config:
          - name: desktops
            vlan_id: 20
          - name: servers
            vlan_id: 30
          - name: printers
            vlan_id: 40
          - name: DMZ
            vlan_id: 50

The Ansible Playbook ran just like before with changed=0. Can we tell if it removed the artisanal_vlan 100?

rtr2#show vlan
VLAN  Name                             Status    Ports
----- -------------------------------- --------- -------------------------------
1     default                          active
20    desktops                         active
30    servers                          active
40    printers                         active
50    DMZ                              active
100   artisanal_vlan                   active

Nope! The goal of resource modules is to update existing resources to match the existing data model. Since our data model (the key, value pairs that represent the VLANs, which are passed under the config parameter in the playbook) only includes VLANs 20, 30, 40 and 50 the eos_vlans module only updates parameters relevant to those particular VLANs.

Why would I use this versus a merged? The major difference between a merged and a replaced is that a merged just makes sure the commands are present that are represented within the data model, whereas the replaced parameter makes your resource match the data model. Let's look at the eos_vlans module to see what it considers as part of the vlans resource.

There are three parameters currently used for the vlans resource:

  • name
  • state (active or suspend)
  • vlan_id (range between 1-4094)

Let's look at the following example:

  Data Model Sent


- name: desktops
  vlan_id: 200
  Existing Arista Config 

vlan 200
   state suspend
!

This is how merged compares to replaced:

 merged

vlan 200
  name desktops
  state suspend
!
replaced


vlan 200
   name desktops
!

The replaced parameter enforces the data model on the network device for each configured VLAN.  In the example above it will remove the `state suspend` because it is not within the data model.  To think of this another way, the replaced parameter is aware of commands that shouldn’t be there as well as what should.

 

Using the overridden state parameter

What happens if I change the state parameter to overridden?  Just change the original example to the following:

---
- name: add vlans
  hosts: arista
  gather_facts: false
  tasks:
    - name: add VLAN configuration
      eos_vlans:
        state: overridden
        config:
          - name: desktops
            vlan_id: 20
          - name: servers
            vlan_id: 30
          - name: printers
            vlan_id: 40
          - name: DMZ
            vlan_id: 50

Now run the Ansible Playbook:

Sean Blog 2

The Ansible Playbook now has changed=1.  But did it remove the artisanal_vlan 100?

Logging into one of the Arista devices confirms it did!

rtr2#show vlan
VLAN  Name                             Status    Ports
----- -------------------------------- --------- -------------------------------
1     default                          active
20    desktops                         active
30    servers                          active
40    printers                         active
50    DMZ                              active

The overridden parameter will enforce all vlans resources to the data model.  This means it removes VLANs that are not defined in the data model being sent.

 

Takeaways

There are currently three ways to push configuration using resource modules.  These are the merged, replaced and overridden parameters. These allow much more flexibility for network engineers to adopt automation in incremental steps.  We realize that most folks will start with the merged parameter as they gain familiarity with the new resource module concepts. Over time organizations will move towards the overridden parameter as they adopt a standard SoT (source of truth) for their data models, wherever they reside.

Where to go next?

I hope you will go try out some of the vlans modules (e.g. ios_vlans for Cisco IOS, junos_vlans for Juniper Junos, etc!)

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Topics:
Networks


 

Sean Cavanaugh

Sean is a Principal Product Marketing Manager, Ansible, where he brings over 10 years of experience building and automating computer networks. Sean previously worked for both Cumulus Networks and Cisco Systems where he helped customers deploy, manage and automate their network infrastructure. He resides in Chapel Hill, NC with his wife and children and tweets from @IPvSean.


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