DevNet Lab 15 – Use Ansible to Back Up and Configure a c8000v Router

DevNet Lab 15 – Use Ansible to Back Up and Configure a c8000v Router

Scenario

In this lab, you will explore the basics of using Ansible to automate backup and interface addressing tasks. First, you will configure Ansible in your DevNet virtual machine. Next, you will use Ansible to connect to the virtual router and back up its configuration. Then you will configure this virtual router with IPv6 addressing and discover that idempotence in automation comes at a significant cost.

Lab Topology

Objectives

After completing the manipulation steps in this document, you will be able to:

Configure Ansible in a DevNet virtual machine Environment
Establish SSH connectivity between the DevNet VM and a virtual router
Automate router configuration tasks using Ansible playbooks
Apply Ansible to configure IPv6 addressing on a router interface
Understand and implement idempotency in Ansible playbooks
Refactor playbook tasks to achieve better idempotency

Part 1: Configure Ansible on the Devnet virtual machine

In this part, you will configure Ansible to run from a specific directory.

Step 1: Create the Ansible directory and configuration file

Ensure the ~/labs/lab15 directory exist and navigate to this folder
```
mkdir -p ~/labs/lab15 && cd ~/labs/lab15
```
Install ansible Python virtual environement

There are two main ways to set up a new Ansible workspace. Packages and Python virtual environments are both viable options. Here we choose to install Ansible in a Python virtual environment to take advantage of the latest release.

We start by creating a requirements.txt file.
```
cat << EOF > requirements.txt
ansible
ansible-lint
ansible-pylibssh
netaddr
EOF
```
Then we install the tools in a virtual environment called ansible.
```
touch .gitignore
if ! grep -q ansible .gitignore; then
    echo ansible >>.gitignore
fi

python3 -m venv ansible 
source ./ansible/bin/activate
pip3 install -r requirements.txt
```

Create a new ansible.cfg file in the lab15 directory from the shell prompt

cat << 'EOF' > ansible.cfg
# config file for Lab 14 virtual router management
[defaults]
# Use inventory/ folder files as source
inventory=inventory/
host_key_checking = False # Don't worry about RSA Fingerprints
retry_files_enabled = False # Do not create them
deprecation_warnings = False # Do not show warnings
interpreter_python = /usr/bin/python3
[inventory]
enable_plugins = auto, host_list, yaml, ini, toml, script
[persistent_connection]
command_timeout=100
connect_timeout=100
connect_retry_timeout=100
ssh_type = libssh
EOF

Ensure that all necessary libraries and collections are up to date.

Update Cisco Ansible collection to the lastest version

ansible-galaxy collection install -f cisco.ios --upgrade

The local collections have priority over the system collection.

ansible-galaxy collection list cisco.ios




# /home/etu/.ansible/collections/ansible_collections
Collection Version
---------- -------
cisco.ios  10.1.0

Create the inventory directory
```
mkdir ~/labs/lab15/inventory
```
The contents of this directory will later be supplemented by the virtual router’s connection parameters.

Step 2: Check SSH access from Devnet VM to virtual router

We start with a shell test connection before to set the configuration for ansible.

Be sure the virtual router is already up and running. If it’s not the case, refer to DevNet Lab 14 – Run the Cisco IOS-XE router VM

One more time, be sure to change placeholders to match your resource allocation.

Here is a sample declaration file for programming hypervisor switch ports:
















ovs:
  switches:
    - name: dsw-host
      ports:
        - name: tapXX7    # management interface connection
          type: OVSPort
          vlan_mode: access
          tag: VVV        # out-of-band VLAN id
        - name: tapXX8    # in-band interface connection
          type: OVSPort
          vlan_mode: trunk
          trunks: [300, 301, 302]
        - name: tapXX9    # unused interface in this lab
          type: OVSPort
          vlan_mode: access
          tag: 999

From the hypervisor shell, apply this switch configuration with the following command:

switch-conf.py lab15-switch.yaml

Return to the DevNet development virtual machine shell, start an initial SSH connection to the virtual router, and accept the router’s fingerprint.

ssh etu@fe80::faad:caff:fefe:XXX%enp0s1








The authenticity of host 'fe80::faad:caff:fefe:XXX%enp0s1 (fe80::faad:caff:fefe:XXX%enp0s1)' can't be established.
RSA key fingerprint is SHA256:e+oyegX4BzzQSKNVz7vjoi8psHTxUjIw/rc/44Y8tHY.
This key is not known by any other names.
Are you sure you want to continue connecting (yes/no/[fingerprint])? yes
Warning: Permanently added 'fe80::faad:caff:fefe:2%enp0s1' (RSA) to the list of known hosts.
(etu@fe80::faad:caff:fefe:XXX%enp0s1) Password: 

rtrXXX#

This SSH connection uses the default user account created by the Zero Touch Programming (ZTP) Python script. This initial router configuration only occurs when the router management interface is connected to the out-of-band auto-addressing VLAN.

Step 3: Create a new Ansible vault file

Create a new vault file called .lab_passwd.yml and enter the unique vault password which will be used for all users passwords to be stored.
```
ansible-vault create $HOME/.lab_passwd.yml
```
```
New Vault password:
Confirm New Vault password:
```
This will open the default editor which is defined by the $EDITOR environment variable.
There we enter a variable name which will designate the password for Web server VM user account.
```
ansible_user_passwd: 4n51bl3
```

Create the ansible_user account on the IOS XE system of the C8000v virtual router

From the default user account SSH connection already opened, add a new user account with the highest privilege level and the same password as the one chosen for the ansible vault.

rtrXXX#conf terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
rtrXXX(config)#user ansible_user privilege 15 secret 4n51bl3
rtrXXX(config)#^Z
rtrXXX#copy run start
Destination filename [startup-config]? 
Building configuration...
[OK]

Here is another copy of the user creation instructions. This version does not include the prompt or messages.

conf terminal
user ansible_user privilege 15 secret 4n51bl3
end
copy running-config startup-config

Close the default user SSH connection and open a new one with the identity ansible_user and list the IPv6 addresses of this router to fill the inventory file.

First, complete your SSH config file in order to avoid identity conflicts on Local Link IPv6 addresses







cat << EOF >> $HOME/.ssh/config

Host fe80::*
	CheckHostIP no
	StrictHostKeyChecking no
	UserKnownHostsFile=/dev/null
EOF

Second, open the new SSH conection

ssh ansible_user@fe80::faad:caff:fefe:XXX%enp0s1
Warning: Permanently added 'fe80::faad:caff:fefe:2%enp0s1' (RSA) to the list of known hosts.
(ansible_user@fe80::faad:caff:fefe:2%enp0s1) Password: 

rtrXXX#sh users
    Line       User       Host(s)              Idle       Location
*434 vty 0     ansible_us idle                 00:00:00 FE80::BAAD:CAFF:FEFE:0

  Interface    User               Mode         Idle     Peer Address

rtrXXX#sh ipv6 int br GigabitEthernet 1
GigabitEthernet1       [up/up]
    FE80::FAAD:CAFF:FEFE:XXX
    2001:678:3FC:VVV:FAAD:CAFF:FEFE:XXX

In the screenshot above, we have a choice of 2 IPv6 addresses: the local link address or the GUA address.

It is good practice to have the DevNet VM and the out-of-band router interface on the same VLAN.
This is why we chose to use the Local Link IPv6 address in the inventory file.

Step 4: Create the Ansible inventory file

Ansible uses an inventory file called hosts, which contains device information used by Ansible playbooks.

In this lab, you will be running Ansible from the git project’s lab directory. Therefore, you will need separate hosts and ansible.cfg files for each lab.

The terms “hosts file” and “inventory file” are synonymous and are used interchangeably throughout the Ansible labs.

The Ansible inventory file defines the devices and groups of devices used by the Ansible playbook. The file can be in one of many formats, including YAML and INI, depending on your Ansible environment.

The inventory file can list devices by IP address or fully qualified domain name (FQDN), and can also include host-specific parameters.

Create the hosts.yml inventory file in the inventory directory, add the following content to the file and save it. Any XXX tags must be edited to identify your own C8000v virtual router instance.

cat << EOF > inventory/hosts.yml
ios:
  hosts:
    rtrXXX:
      ansible_host: fe80::faad:caff:fefe:XXX%enp0s1
      ansible_port: 2222
  vars:
    ansible_ssh_user: ansible_user
    ansible_ssh_pass: "{{ ansible_user_passwd }}"
    ansible_connection: network_cli
    ansible_network_os: ios

all:
  children:
    ios:
EOF

The hosts.yml file starts with the [ios] section. This section lists aliases for a set of devices. Here we have a single alias: rtrXXX. An host group is used in the Ansible playbook to refer to a device specified by the ansible_host and ansible_port variables.

Within the [ios] section, the hosts.yml file specifies a set of variables that the Ansible playbook will use to access the device. These are the SSH credentials that Ansible needs to securely access the c8000v virtual router.

ansible_ssh_user: Username used to connect to the remote device. Without this, the user that is running the ansible playbook would be used.
ansible_ssh_pass: Password for ansible_ssh_user. If omitted, the SSH key would be used.
ansible_connection: Specifies the library to use to connect to the device.
ansible_network_os:: Specifies the operating system used on the endpoint.

Step 5: Verify the Ansible configuration and check router access.

Use the ansible --version command to display version information.

ansible --version









ansible [core 2.18.6]
  config file = /home/etu/labs/lab15/ansible.cfg
  configured module search path = ['/home/etu/.ansible/plugins/modules', '/usr/share/ansible/plugins/modules']
  ansible python module location = /home/etu/labs/lab15/ansible/lib/python3.13/site-packages/ansible
  ansible collection location = /home/etu/.ansible/collections:/usr/share/ansible/collections
  executable location = /home/etu/labs/lab15/ansible/bin/ansible
  python version = 3.13.3 (main, Apr  8 2025, 19:55:40) [GCC 14.2.0] (/home/etu/labs/lab15/ansible/bin/python3)
  jinja version = 3.1.6
  libyaml = True

Parse your own ansible.cfg file.

Now, you need to edit your ansible.cfg file to look at the location of your hosts.yml inventory file.

Open the ansible.cfg file and search lines referring to inventory.
```
grep -A2 inventory ansible.cfg
```
```
# Use inventory/ folder files as source
inventory=inventory/
host_key_checking = False # Don't worry about RSA Fingerprints
retry_files_enabled = False # Do not create them
--
[inventory]
enable_plugins = auto, host_list, yaml, ini, toml, script
[persistent_connection]
```
As in Python, the # is used for comments within the ansible.cfg file.

Comments cannot follow an entry. Ansible treats the # and subsequent comment as part of the filename. Therefore, in these cases, the # comment must be on a separate line.

However, variables can have a comment on the same line, as shown for host_key_checking and retry_files_enabled.

The ansible.cfg file tells Ansible where to find the inventory file and sets certain default parameters. The information you put in your ansible.cfg file is:

inventory=inventory/

All your inventory files are in the inventory directory.

host_key_checking = False

There are no SSH keys set up in the local development environment. You have set host_key_checking to False, which is the default. In a production network, host_key_checking would be set to True.

retry_files_enabled = False

If Ansible has troubles running playbooks for a host, it will output the host name to a file in the current directory ending in retry. To avoid clutter, it is common to disable this setting.

Step 6: Ansible configuration files summary

In this part you have configured Ansible to run in the lab directory.

In this lab you will need an ansible.cfg file in your ansible directory and an inventory file hosts.yml in the inventory directory.

You edited the hosts file to contain login and IP address information for the virtual router
You edited the ansible.cfg file to use the local hosts file as the inventory file

In the next part, you will create a playbook to tell Ansible what to do.

The inventory status can be checked using the ansible-inventory command:

ansible-inventory --yaml --list











all:
  children:
    ios:
      hosts:
        rtrXXX:
          ansible_connection: network_cli
          ansible_host: fe80::faad:caff:fefe:XXX%enp0s1
          ansible_network_os: ios
          ansible_port: 2222
          ansible_ssh_pass: "{{ ansible_user_passwd }}"
          ansible_ssh_user: ansible_user

The connection to the router device can be checked using the Ansible ping module:

ansible -m ping all --ask-vault-pass --extra-vars '@$HOME/.lab_passwd.yml'





Vault password: 
rtrXXX | SUCCESS => {
    "changed": false,
    "ping": "pong"
}

Part 2: Use Ansible to back up router configuration

In this part, you will create an Ansible playbook that automates the process of backing up the router configuration. Playbooks are at the heart of Ansible. Whenever you want Ansible to retrieve information or perform an action on a device or group of devices, you run a playbook to get the job done.

An Ansible playbook is a YAML file containing one or more plays. Each play is a collection of tasks.

play: A matching set of tasks to a device or group of devices.
task: A single action that references a module to be executed along with any input arguments and actions. These tasks can be simple or complex, depending on the need for permissions, the order in which the tasks are executed, and so on.

A playbook may also contain roles. A role is a mechanism for splitting a playbook into multiple components or files, simplifying the playbook and making it easier to reuse. For example, the common role is used to store tasks that can be used in all of your playbooks.
Roles are beyond the scope of this lab.

The Ansible YAML playbook includes objects, lists and modules.

A YAML object consists of one or more key-value pairs. Key-value pairs are separated by a colon without quotation marks, for example hosts: Router.
An object can contain other objects, such as a list. YAML uses lists or arrays. A hyphen “-” is used for each element in the list.
Ansible ships with a set of modules (called the module library) that can be run directly on remote hosts or through playbooks.
An example is the ios_command module, which is used to send commands to an IOS device and return the results. Each task typically consists of one or more Ansible modules.

The ansible-playbook command uses parameters to specify

The vault file that contains the credentials to decrypt to connect to the user account specified in the inventory file.
The playbook you want to run backup_router_playbook.yml.

Step 1: Create your Ansible playbook.

The Ansible playbook is a YAML file. Make sure you use the correct YAML indentation. Every space and hyphen is important. You may lose some formatting if you copy and paste the code in this lab.

Create a new file in the ansible directory with the following name: backup_router_playbook.yml
Add the following information to the file.










# The purpose of this playbook is to automatically back up the running
# configuration of a Cisco router.
---
- name: AUTOMATIC BACKUP OF ROUTER CONFIGURATION
  hosts: ios
  tasks:
    - name: BACKUP RUNNING CONFIG
      cisco.ios.ios_config:
        backup: true
...

Step 2: Examine your Ansible playbook.

The playbook you have created contains one play with one task. The following is an explanation of your playbook:

---: This is at the beginning of every YAML file and tells YAML that this is a separate document. Each file can contain several documents, separated by ---.
name:: This is the name of the play.
hosts: ios: This is the alias previously configured in the hosts.yml file. By referencing this alias in your playbook, the playbook can use any parameters associated with this inventory file entry, including the IP address of the devices.
tasks:: This keyword specifies one or more tasks to perform.
The task is to backup the router configuration.
cisco.ios.ios_config:: This is an Ansible module that is used to manage an IOS device configuration. The ios_config module belongs to the cisco.ios collection.

In the Linux terminal, you can use the ansible-doc module_name command to view the manual pages for any module and the parameters associated with that module. (e.g. ansible-doc cisco.ios.ios_command or ansible-doc cisco.ios.ios_config).
backup:: This parameter is associated with the ios_config module. It is used to list IOS commands in the playbook that are to send to the remote IOS device. The resulting command output is returned.

Step 3: Run the Ansible backup Playbook.

Now you can run the Ansible playbook using the ansible-playbook command:

ansible-playbook backup_router_playbook.yml --ask-vault-pass --extra-vars '@$HOME/.lab_passwd.yml'
Vault password:










PLAY [AUTOMATIC BACKUP OF ROUTER CONFIGURATION] ***************************************

TASK [Gathering Facts] ****************************************************************
ok: [rtrXXX]

TASK [BACKUP RUNNING CONFIG] **********************************************************
changed: [rtrXXX]

PLAY RECAP ****************************************************************************
rtrXXX   : ok=2    changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

The PLAY RECAP should display ok=2 changed=1 indicating a successful playbook execution.

If your Ansible playbook fails, check the following items in your playbook:

Ensure that your hosts.yml and ansible.cfg files are correct.
Ensure the YAML indentation is correct.
Ensure that your IOS command is correct.
Check the syntax of the entire Ansible playbook.
Verify that you can ping the router.

If you continue to have problems, check the content of the inventory file content with the ansible-inventory command:

ansible-inventory --yaml --list











all:
  children:
    ios:
      hosts:
        rtrXXX:
          ansible_connection: network_cli
          ansible_host: fe80::faad:caff:fefe:XXX%enp0s1
          ansible_network_os: ios
          ansible_port: 2222
          ansible_ssh_pass: "{{ ansible_user_passwd }}"
          ansible_ssh_user: ansible_user

Check your playbook syntax with the ansible-lint command:

ansible-lint backup_router_playbook.yml


Passed: 0 failure(s), 0 warning(s) on 1 files. Last profile that met the validation criteria was 'production'.

The command results will show you the lines where there is a syntax error.

Step 4: Verify the backup file has been created.

List the files in the backup folder and open the most recent one. You can also use the head command to print the first few lines of the backup file. You now have a backup of the router configuration.

ls -A backup


rtrXXX_config.2025-06-11@15:01:26

head -n 20 backup/rtrXXX_config.2025-06-11@15\:01\:26




















Building configuration...

Current configuration : 6841 bytes
!
! Last configuration change at 14:47:54 WEST Wed Jun 11 2025 by etu
! NVRAM config last updated at 14:51:27 WEST Wed Jun 11 2025 by etu
!
version 17.16
service timestamps debug datetime msec
service timestamps log datetime msec
platform qfp utilization monitor load 80
platform sslvpn use-pd
platform console serial
!
hostname rtrXXX
!
boot-start-marker
boot-end-marker
!
!

Part 3: Use Ansible to configure a router trunk interface

In this Part, you will create another Ansible playbook to configure IPv6 addressing on the C8000v router.

Step 1: Create a new playbook.

Create a subdirectory named host_vars which will store variables of any device named in the inventory

mkdir host_vars


mkdir: created directory 'host_vars'

Create a new YAML file in the host_vars directory with the device name: rtrXXX.yml



























---
rtr_id: XXX

# The IPv6 addresses are calculated by concatenating the following elements:
# the global prefix, the VLAN ID, the router ID as the interface ID, and the mask.
# For instance, the IPv6 address for VLAN 100 on router 7 is 2001:678:3fc:64::XXX/64.
ipv6_prefix: "2001:678:3fc:"
ipv6_intf_id: "{{ '%x' % rtr_id }}"
ipv6_mask: 64

# The main parent interface is GigabitEthernet2.
# The subinterfaces are created by appending the VLAN ID to the main interface.
# For instance, the subinterface for VLAN 100 is GigabitEthernet2.100.
interface:
  type: GigabitEthernet
  id: 2
  vlans:
    - name: red
      id: 300
      desc: RED VLAN SUBINTERFACE
    - name: purple
      id: 301
      desc: PURPLE VLAN SUBINTERFACE
    - name: blue
      id: 302
      desc: BLUE VLAN SUBINTERFACE

Note that the keyword interface: is singular. It should be plural. For simplicity, we decided to limit the scope of the playbook to a single interface.

Create a new file named router_config_playbook.yml in your lab directory, and add the following tasks to the file. Make sure you use the proper YAML indentation. Every space and dash is significant.



































































































# The purpose of this playbook is to configure IPv6 addresses on subinterfaces
# of a Cisco router.
#
# 1. Calculate IPv6 addresses for each VLAN
# 2. Display the calculated IPv6 addresses, networks, and gateways
# 3. Configure the main parent interface
# 4. Configure sub-interfaces with dot1Q encapsulation, IPv6 addresses, and additional settings
# 5. Save the output of the IPv6 interface brief
# 6. Test reachability to the gateway address of each VLAN
# 7. Save the results of the reachability test
---
- name: VLAN SUBINTERFACES CONFIGURATION
  hosts: all

  tasks:
    - name: CALCULATE IPv6 ADDRESSES
      ansible.builtin.set_fact:
        calculated_vlans: >
          {{
            calculated_vlans | default([]) +
            [
              item | combine(
                {
                  'addr': ipv6_prefix ~
                          ('%x' % item.id) ~
                          '::' ~
                          ipv6_intf_id ~
                          '/' ~
                          ipv6_mask
                }
              )
            ]
          }}
      loop: "{{ interface.vlans }}"
      loop_control:
        label: "{{ item.name }}"

    - name: VARS ANALYSIS
      ansible.builtin.debug:
        msg:
          - "vlan: {{ item.id }} --> address: {{ item.addr }}"
          - "Network: {{ item.addr | ansible.utils.ipaddr('network') }}/{{ item.addr | ansible.utils.ipaddr('prefix') }}"
          - "Gateway: {{ item.addr | ansible.utils.ipaddr('1') }}"
      loop: "{{ calculated_vlans }}"

    - name: MAIN PARENT INTERFACE CONFIG
      cisco.ios.ios_interfaces:
        config:
          - name: "{{ interface.type }}{{ interface.id }}"
            description: IPV6 ANSIBLE PLAYBOOK CONFIGURATION
            enabled: true

    - name: SUB INTERFACES CONFIG
      cisco.ios.ios_config:
        lines:
          - description {{ item.desc }}
          - encapsulation dot1Q {{ item.id }}
          - ipv6 address {{ item.addr }}
          - ipv6 enable
          - ipv6 nd ra suppress all
        parents:
          - interface {{ interface.type }}{{ interface.id }}.{{ item.id }}
        match: exact
        replace: block
      with_items: "{{ calculated_vlans }}"

    - name: SHOW IPv6 INTERFACE BRIEF
      cisco.ios.ios_command:
        commands:
          - show ipv6 interface brief
      register: output

    - name: ENSURE TRACE DIRECTORY EXISTS
      delegate_to: localhost
      ansible.builtin.file:
        path: trace
        state: directory
        mode: "0755"

    - name: SAVE OUTPUT
      ansible.builtin.copy:
        content: "{{ output.stdout[0] }}"
        dest: "trace/ipv6_int_brief_{{ inventory_hostname }}.txt"
        mode: "0644"

    - name: REACHABILITY TEST
      cisco.ios.ios_ping:
        dest: "{{ item.addr | ansible.utils.ipaddr('1') | ansible.utils.ipaddr('address') }}"
        afi: ipv6
      loop: "{{ calculated_vlans }}"
      register: ping_results

    - name: SAVE PING RESULTS
      delegate_to: localhost
      ansible.builtin.copy:
        content: "{{ ping_results.results | to_nice_json }}"
        dest: "trace/ipv6_ping_{{ inventory_hostname }}.txt"
        mode: "0644"

Step 2: Examine the Ansible playbook.

The main purpose of this playbook is to illustrate variable formatting and calculations. A set of VLAN identifiers is defined from the contents of the host_vars/rtrXXX.yml file. Then IPv6 subinterface addresses are calculated using a combination of:

The prefix stands for the network part
The VLAN id stands for the subnetwork part
The router id stands for the host part

Note that we only need to change the router and VLAN identifiers to have all the IPv6 addresses automatically calculated.

Now, as we look at the contents of the router_config_playbook.yml file and what it does, here is a brief description of the elements used:

CALCULATE IPv6 ADDRESSES: Iteratively builds a list of enriched VLAN objects by adding a calculated IPv6 address to each VLAN entry using the hexadecimal VLAN ID in the network portion and the router ID in the host portion, following a standard IPv6 address format with prefix and mask.
VARS ANALYSIS: Use the debug module to view the calculation results as a detailed network address plan.
MAIN PARENT INTERFACE CONFIG: Use the ios_interfaces module to enable the parent network interface and set its description.
SUB INTERFACES CONFIG: Use the ios_config module to send line-by-line configuration instructions for each VLAN subinterface. This way of configuring network interfaces is useful when using commands that are not provided by the Ansible libraries. The downside here is that we lose idempotency and a warning is sent each time the playbook is run.
You can use the ansible-doc cisco.ios.ios_config command to see the details for the parents and match parameters used in this playbook.
SHOW IPv6 INTERFACE BRIEF: Use the ios_command module to send the show ipv6 interface brief command. The output of the command is registered in an Ansible variable called output.
ENSURE TRACE DIRECTORY EXISTS: Creates the trace directory in the DevNet lab tree if necessary.
SAVE OUTPUT: Use the copy module to save the contents of the outpout variable to a file located in the trace directory on the Devnet VM.
REACHABILITY TEST: Use the ios_ping module to send ICMPv6 requests to the network gateway of each subinterface VLAN.

Step 3: Execute the Ansible playbook to set up IPv6 addressing on the virtual router

Now you can run the Ansible playbook with the ansible-playbook command. The -vvv verbose option can be added to display the tasks being performed in the playbook.

Do not forget to replace the rtr_id value in the host_vars/rtrXXX.yaml with your own out of band tap interface number!.
This is mandatory in order to avoid duplicate addresses on different virtual routers.

ansible-playbook router_config_playbook.yml --ask-vault-pass --extra-vars '@$HOME/.lab_passwd.yml'
Vault password:





























































PLAY [VLAN SUBINTERFACES CONFIGURATION] ******************************************

TASK [Gathering Facts] ***********************************************************
ok: [rtrXXX]

TASK [CALCULATE IPv6 ADDRESSES] **************************************************
ok: [rtrXXX] => (item=red)
ok: [rtrXXX] => (item=purple)
ok: [rtrXXX] => (item=blue)

TASK [VARS ANALYSIS] *************************************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'}) => {
    "msg": [
        "vlan: 300 --> address: 2001:678:3fc:12c::XXX/64",
        "Network: 2001:678:3fc:12c::/64",
        "Gateway: 2001:678:3fc:12c::1/64"
    ]
}
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'}) => {
    "msg": [
        "vlan: 301 --> address: 2001:678:3fc:12d::XXX/64",
        "Network: 2001:678:3fc:12d::/64",
        "Gateway: 2001:678:3fc:12d::1/64"
    ]
}
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'}) => {
    "msg": [
        "vlan: 302 --> address: 2001:678:3fc:12e::XXX/64",
        "Network: 2001:678:3fc:12e::/64",
        "Gateway: 2001:678:3fc:12e::1/64"
    ]
}

TASK [MAIN PARENT INTERFACE CONFIG] **********************************************
ok: [rtrXXX]

TASK [SUB INTERFACES CONFIG] *****************************************************
changed: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
changed: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
changed: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})
[WARNING]: To ensure idempotency and correct diff the input configuration lines should be similar to how they appear if present in the running configuration on device

TASK [SHOW IPv6 INTERFACE BRIEF] *************************************************
ok: [rtrXXX]

TASK [ENSURE TRACE DIRECTORY EXISTS] *********************************************
ok: [rtrXXX -> localhost]

TASK [SAVE OUTPUT] ***************************************************************
ok: [rtrXXX]

TASK [REACHABILITY TEST] *********************************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})

TASK [SAVE PING RESULTS] *********************************************************
changed: [rtrXXX -> localhost]

PLAY RECAP ***********************************************************************
rtrXXX                     : ok=10   changed=2    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

All of these playbook tasks have been successfully completed. The ok counter has reached 10, which means that all 10 tasks have been completed.

The changed counter is 2, meaning 2 changes have been made. If we look for the changed keyword in the playbook run screenshot above, we can identify these configuration updates.

IPv6 addresses have been set for the three VLANs declared in the router’s host_vars file.
Ping results have been saved as artifacts that prove network communications are functional.

Step 4: Verify the configuration trace file has been created.

You can view the contents of the trace file with cat trace/ipv6_int_brief_Router.txt. You now have a trace of the virtual router’s interface and subinterface configuration.

cat trace/ipv6_int_brief_rtrXXX.txt
















GigabitEthernet1       [up/up]
    FE80::FAAD:CAFF:FEFE:XXX
    2001:678:3FC:34:FAAD:CAFF:FEFE:XXX
GigabitEthernet2       [up/up]
    unassigned
GigabitEthernet2.300   [up/up]
    FE80::FAAD:CAFF:FEFE:YYY
    2001:678:3FC:12C::XXX
GigabitEthernet2.301   [up/up]
    FE80::FAAD:CAFF:FEFE:YYY
    2001:678:3FC:12D::XXX
GigabitEthernet2.302   [up/up]
    FE80::FAAD:CAFF:FEFE:YYY
    2001:678:3FC:12E::XXX
GigabitEthernet3       [administratively down/down]
    unassigned

Part 4: Examining the idempotency claims of Ansible playbook tasks

And here comes the devil!

When we try to run the playbook again, we notice that the changed counter is always 2. This shouldn’t be the case because the VLAN subinterfaces already have IPv6 addresses set.
Applying not-so-new addresses each time the playbook runs can cause communications to break.

[WARNING]: To ensure idempotency and correct diff the input configuration lines
should be similar to how they appear if present in the running configuration on device

When we dig deeper into the IOS configuration lines, no matter how hard we try, we always get the warning. So, the indemptency claim is broken.

Step 1: Analyzing why idempotency is not achieved as expected

In our router_config_playbook.yml playbook, we chose to use the cisco.ios.ios_config, which sends lines of IOS XE instructions in configuration mode. In addition, the replace: block statement should ensure an idempotent configuration by replacing the entire specified configuration block for each sub-interface rather than attempting to merge individual lines. This should ensure that the interface configuration matches exactly what’s defined in the playbook.













- name: SUB INTERFACES CONFIG
  cisco.ios.ios_config:
    lines:
      - description {{ item.desc }}
      - encapsulation dot1Q {{ item.id }}
      - ipv6 address {{ item.addr }}
      - ipv6 enable
      - ipv6 nd ra suppress all
    parents:
      - interface {{ interface.type }}{{ interface.id }}.{{ item.id }}
    match: exact
    replace: block
  with_items: "{{ calculated_vlans }}"

The problem we’re encountering is symptomatic of using the syntax of a network device configuration language like IOS, which was not designed for automation in the first place.

There are two possible solutions to overcome this:

abandon the configuration language and use only the APIs
Break the task into multiple subtasks using elementary modules to ensure idempotency at an almost atomic level.

Since using APIs for our virtual router configuration is beyond the scope of this lab, we choose the latter.

Step 2: Refactor the playbook to achieve indempotency

Here is a new block of 3 tasks, replacing the previous single SUB INTERFACES CONFIG task, each using a specific module to achieve idempotent processing.




































    - name: CONFIGURE IPv6 ADDRESSES ON SUB-INTERFACES
      block:
        - name: CREATE SUB-INTERFACES
          cisco.ios.ios_interfaces:
            config:
              - name: "{{ interface.type }}{{ interface.id }}.{{ item.id }}"
                description: "{{ item.desc }}"
                enabled: true
            state: merged
          loop: "{{ calculated_vlans }}"

        - name: CONFIGURE DOT1Q ENCAPSULATION AND ADDITIONAL IPv6 SETTINGS
          cisco.ios.ios_config:
            lines:
              - encapsulation dot1Q {{ item.id }}
              - ipv6 enable
              - ipv6 nd ra suppress all
            parents: "interface {{ interface.type }}{{ interface.id }}.{{ item.id }}"
          loop: "{{ calculated_vlans }}"

        - name: CONFIGURE IPv6 ADDRESSES ON SUB-INTERFACES
          cisco.ios.ios_l3_interfaces:
            config:
              - name: "{{ interface.type }}{{ interface.id }}.{{ item.id }}"
                ipv6:
                  - address: "{{ item.addr | ansible.utils.ipaddr('address') }}/{{ item.addr | ansible.utils.ipaddr('prefix') }}"
            state: merged
          loop: "{{ calculated_vlans }}"
      rescue:
        - name: Log configuration failure
          ansible.builtin.debug:
            msg: Failed to configure IPv6 on sub-interfaces. Check interface status and configuration.

        - name: Notify on failure
          ansible.builtin.fail:
            msg: IPv6 interface configuration failed. See previous messages for details.

cisco.ios.ios_interfaces: Manages the interface configuration on Cisco IOS devices. Define, modify, or remove physical and logical interfaces with attributes such as descriptions, MTU settings, and administrative state (enabled/disabled).
cisco.ios.ios_l3_interfaces: Manages Layer 3 interface configurations on Cisco IOS devices, allowing for the declarative configuration of IPv4 and IPv6 addresses with appropriate subnet masks or prefixes using state-based operations (merged, replaced, overridden, or deleted).
cisco.ios.ios_config: Pushes configuration commands to Cisco IOS devices, supporting parent-child relationships for hierarchical commands, various matching strategies (line, strict, exact), and configuration replacement methods (line, block) to manage device configurations.

As we can see, the cisco.ios.ios_config module is a last-resort solution when no other module in the collection provides the required configuration options.

Why use a task block?: Using a block in this playbook allows for logical grouping of related tasks for sub-interface configuration, provides structured error handling with the rescue clause, and ensures that if any step in the configuration process fails, the playbook can gracefully report the error and take appropriate recovery actions without leaving interfaces in an inconsistent state.

Step 3: Run the edited playbook en verify idempotency

Start by replacing the orignal SUB INTERFACES CONFIG task with the suggested block of three subtasks. Then run the playbook again and look at the changed counter value.

Here is an excerpt of the playbook run focusing on the three subinterfaces configuration processing.

PLAY [VLAN SUBINTERFACES CONFIGURATION] ***************************************************

TASK [Gathering Facts] ********************************************************************
ok: [rtrXXX]

TASK [CALCULATE IPv6 ADDRESSES] ***********************************************************
ok: [rtrXXX] => (item=red)
ok: [rtrXXX] => (item=purple)
ok: [rtrXXX] => (item=blue)

TASK [VARS ANALYSIS] **********************************************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'}) => {
    "msg": [
        "vlan: 300 --> address: 2001:678:3fc:12c::XXX/64",
        "Network: 2001:678:3fc:12c::/64",
        "Gateway: 2001:678:3fc:12c::1/64"
    ]
}
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'}) => {
    "msg": [
        "vlan: 301 --> address: 2001:678:3fc:12d::XXX/64",
        "Network: 2001:678:3fc:12d::/64",
        "Gateway: 2001:678:3fc:12d::1/64"
    ]
}
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'}) => {
    "msg": [
        "vlan: 302 --> address: 2001:678:3fc:12e::XXX/64",
        "Network: 2001:678:3fc:12e::/64",
        "Gateway: 2001:678:3fc:12e::1/64"
    ]
}

TASK [MAIN PARENT INTERFACE CONFIG] *******************************************************
ok: [rtrXXX]

TASK [CREATE SUB-INTERFACES] **************************************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})

TASK [CONFIGURE DOT1Q ENCAPSULATION AND ADDITIONAL IPv6 SETTINGS] *************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})

TASK [CONFIGURE IPv6 ADDRESSES ON SUB-INTERFACES] *****************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})

TASK [SHOW IPv6 INTERFACE BRIEF] **********************************************************
ok: [rtrXXX]

TASK [ENSURE TRACE DIRECTORY EXISTS] ******************************************************
ok: [rtrXXX -> localhost]

TASK [SAVE OUTPUT] ************************************************************************
ok: [rtrXXX]

TASK [REACHABILITY TEST] ******************************************************************
ok: [rtrXXX] => (item={'name': 'red', 'id': 300, 'desc': 'RED VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12c::XXX/64'})
ok: [rtrXXX] => (item={'name': 'purple', 'id': 301, 'desc': 'PURPLE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12d::XXX/64'})
ok: [rtrXXX] => (item={'name': 'blue', 'id': 302, 'desc': 'BLUE VLAN SUBINTERFACE', 'addr': '2001:678:3fc:12e::XXX/64'})

TASK [SAVE PING RESULTS] ******************************************************************
changed: [rtrXXX -> localhost]

PLAY RECAP ********************************************************************************
rtrXXX                     : ok=12   changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

We win! There is no warning message or change indicator when network interfaces have their IPv6 addresses already configured.

If we look at the playbook recap, the changed counter value is 1 because we saved the ping results as new artifacts.

PLAY RECAP ***********************************************************************
rtrXXX                     : ok=12   changed=1    unreachable=0    failed=0    skipped=0    rescued=0    ignored=0

We can now conclude this part by acknowledging that indempotency in Ansible playbooks comes at a cost. When designing tasks, we need to think in terms of fine-grained tuning.

Conclusion

This lab provides a practical introduction to network automation using Ansible, with a focus on both functionality and idempotency. Through hands-on exercises, you configured Ansible, backed up router configurations, and automated IPv6 addressing on router interfaces.

By creating and refining playbooks, you experienced the process of developing efficient and idempotent automation scripts. The lab highlighted the challenges of achieving true idempotency when working with network device configuration languages, and demonstrated how to overcome these challenges by breaking down tasks and using specialized Ansible modules.

You learned that while Ansible claims to provide idempotent operations, achieving this in practice often requires careful consideration and sometimes refactoring of playbook tasks. The lab showed how to analyze playbook execution, identify non-idempotent behavior, and implement solutions to ensure that repeated playbook runs produce consistent and predictable results.

This experience serves as a foundation for more advanced network automation projects, emphasizing the importance of not just automating tasks, but doing so in a way that is reliable, repeatable, and efficient. The skills acquired in this lab will be valuable as you continue to explore and implement network automation strategies in more complex environments.