Getting Started with Python Network Libraries for Network Engineers – Part 1

This blog post will be the first in a series covering common Python libraries that can be used to interact with network devices. In this post we will cover the Netmiko Python library by Kirk Byers. Netmiko is based on the Paramiko Python library, but whereas Paramiko was designed to interact with standard OpenSSH devices (like Linux), Netmiko was designed to interact with network devices. It has a large number of supported platforms included for connecting via SSH, and it can also accommodate limited Telnet or serial connections as well as Secure Copy (SCP) for file transfers.

Installation

You can install Netmiko via pip install netmiko. Or, if you are using Poetry, you can use poetry add netmiko.

Getting Connected

Note: We will only be covering connecting via SSH in this blog post.

Like all SSH connections, Netmiko requires a hostname (IP or DNS name) and an authentication method (generally username and password) to get connected. In addition to this, you will also need to specify the device type you will be connecting to.

>>> from netmiko import ConnectHandler
>>> 
>>> conn = ConnectHandler(
...     host="192.0.2.3",
...     username="cisco",
...     password="cisco",
...     device_type="cisco_ios"
... )

There are two ways of determining the device type: looking it up in a list or having Netmiko try to detect the device type automatically. You can see the list of current device types by digging into the code on GitHub, specifically the CLASS_MAPPER_BASE dictionary in the ssh_dispatcher.py file. If, however, you aren’t exactly sure which device type you need to choose, you can use the SSHDetect class to have Netmiko help:

>>> from netmiko import ConnectHandler, SSHDetect
>>> 
>>> detect = SSHDetect(
...     host="192.0.2.3",
...     username="cisco",
...     password="cisco",
...     device_type="autodetect"  # Note specifically passing 'autodetect' here is required
... )
>>> detect.autodetect()  # This method returns the most likely device type
'cisco_ios'
>>> detect.potential_matches  # You can also see all the potential device types and their corresponding accuracy rating
{'cisco_ios': 99}
>>> conn = ConnectHandler(
...     host="192.0.2.3",
...     username="cisco",
...     password="cisco",
...     device_type=detect.autodetect()
... )

Common Methods

Once you have instantiated your ConnectHandler object, you can send single show commands via the .send_command() method. You will use the same command syntax you would type in if you were directly connected to the device via SSH:

>>> output = conn.send_command("show ip int br")
>>> print(output)
Interface              IP-Address      OK? Method Status                Protocol
FastEthernet0          unassigned      YES NVRAM  down                  down
GigabitEthernet1/0/1   unassigned      YES unset  up                    up
GigabitEthernet1/0/2   unassigned      YES unset  up                    up
GigabitEthernet1/0/3   unassigned      YES unset  up                    up
...

Note: You can send multiple show commands back-to-back with the .send_multiline(["command1", "command2"]).

If you want to run a command to edit the configuration, you would use the .send_config_set() method instead. This method takes care of entering and exiting configuration mode for you, and it requires the commands be in a list or set:

>>> output = conn.send_config_set(("interface Gi1/0/3", "no description"))
>>> print(output)
configure terminal
Enter configuration commands, one per line.  End with CNTL/Z.
cisco(config)#interface Gi1/0/3
cisco(config-if)#no description
cisco(config-if)#end
cisco#

And since we are good network engineers, we know we should always save our configuration after making changes with the .save_config() method:

>>> output = conn.save_config()
>>> print(output)
write mem
Building configuration...
[OK]
cisco#

Command Output Parsing

We’ve covered parsing strategies here on this blog before, including the three currently supported parsers Netmiko supports which are TextFSM, TTP, and Genie. Let’s take a quick look on how to use them with Netmiko.

TextFSM

If you are just starting out, the easiest way to get structured output data would be to use the included TextFSM parser. By default, Netmiko includes the TextFSM library for the parsing as well as NTC Templates to use as the default templates. To get structured output, simply add use_textfsm=True to the parameters of the .send_command() method:

>>> output = conn.send_command("show interfaces", use_textfsm=True)
>>> pprint(output)
[{'abort': '',
  'address': '381c.1ae6.cd81',
  'bandwidth': '100000 Kbit',
  'bia': '381c.1ae6.cd81',
  'crc': '0',
  'delay': '100 usec',
...

You can see the template used for this command here.

TTP

Netmiko also supports the TTP parsing library, but you will need to install it via pip install ttp or poetry add ttp first. It also does not currently include any templates, so you will need to find or create your own and then provide the path to those templates when you send your command.

Creating TTP templates yourself is definitely more of a manual process, but it gives you the freedom to pare down to only the information that you need. For example, if you just need the interface name, status, and description you can have a template like so:

{{ interface }} is {{ link_status }}, line protocol is {{ protocol_status }} {{ ignore }}
  Description: {{ description }}

And then you would reference the template path using ttp_template:

>>> output = conn.send_command("show interfaces", use_ttp=True, ttp_template="templates/show_interfaces.ttp")
>>> pprint(output)
[[[{'description': 'CAM1',
    'interface': 'GigabitEthernet1/0/1',
    'link_status': 'up',
    'protocol_status': 'up'},
   {'description': 'CAM2',
    'interface': 'GigabitEthernet1/0/2',
    'link_status': 'up',
    'protocol_status': 'up'},
...

Genie

The last parser that Netmiko currently supports is Genie. Netmiko, however, does not install Genie nor its required library, PyATS, by default, so you will need to install them separately via pip install 'pyats[library]' or poetry add 'pyats[library]'. Once they are installed, it is again very similar to enable the Genie parsing:

>>> output = conn.send_command("show interfaces", use_genie=True)
>>> pprint(output)
{'GigabitEthernet1/0/1': {'arp_timeout': '04:00:00',
                          'arp_type': 'arpa',
                          'bandwidth': 100000,
                          'connected': True,
                          'counters': {'in_broadcast_pkts': 41240,
...

Note: Genie does not support custom templates.