Python Netmiko — Connect and Run Show Commands

1. How Netmiko Works

Netmiko abstracts the SSH session lifecycle into a clean Python object. Understanding the layers beneath it explains why certain parameters are required and what Netmiko is doing on your behalf:

Common Netmiko device_type Values

2. Lab Environment & Prerequisites

3. Step 1 — Install Python and Netmiko

Verify Python Installation

# ── On Linux / macOS ─────────────────────────────────────────────
$ python3 --version
Python 3.11.4

$ pip3 --version
pip 23.2.1 from /usr/local/lib/python3.11/site-packages/pip (python 3.11)

# ── On Windows (Command Prompt or PowerShell) ─────────────────────
C:\> python --version
Python 3.11.4

C:\> pip --version
pip 23.2.1
  

Install Netmiko

# ── Install Netmiko and its dependencies ─────────────────────────
# Netmiko automatically installs: paramiko, textfsm, ntc-templates,
# scp, pyserial, and other dependencies
$ pip3 install netmiko

Collecting netmiko
  Downloading netmiko-4.3.0-py3-none-any.whl (239 kB)
Collecting paramiko>=2.9.5
Collecting textfsm!=1.1.0,>=1.1.2
Collecting ntc-templates>=2.0.0
Installing collected packages: paramiko, textfsm, ntc-templates, netmiko
Successfully installed netmiko-4.3.0 paramiko-3.3.1 textfsm-1.1.3
ntc-templates-4.1.0

# ── Verify install ────────────────────────────────────────────────
$ python3 -c "import netmiko; print(netmiko.__version__)"
4.3.0

# ── Optional: use a virtual environment (recommended for projects) ─
$ python3 -m venv netmiko-lab
$ source netmiko-lab/bin/activate        # Linux/macOS
$ netmiko-lab\Scripts\activate           # Windows
(netmiko-lab) $ pip install netmiko
  

4. Step 2 — Basic Connection and First Command

The minimal working script: connect to the router, run one command, print the output, disconnect. This is the foundation every more complex script builds on:

# basic_connect.py
# Minimal Netmiko script: connect, run one show command, disconnect

from netmiko import ConnectHandler

# ── Device definition dictionary ─────────────────────────────────
# All connection parameters are passed as a single dictionary
device = {
    'device_type': 'cisco_ios',     # Netmiko driver for Cisco IOS/IOS-XE
    'host':        '192.168.10.1',  # Router management IP
    'username':    'netauto',      # SSH username on the router
    'password':    'AutoPass2026!', # SSH password
    'port':        22,             # SSH port (default 22)
    'timeout':     10,             # TCP connection timeout in seconds
}

# ── Establish the SSH connection ──────────────────────────────────
# ConnectHandler returns a connection object representing the session
print("Connecting to NetsTuts-R1...")
connection = ConnectHandler(**device)
print(f"Connected! Prompt: {connection.find_prompt()}")

# ── Run the show command ──────────────────────────────────────────
# send_command() waits for the device prompt to return
# Returns the command output as a plain Python string
output = connection.send_command("show ip interface brief")

# ── Print the raw output ──────────────────────────────────────────
print("\n--- Raw Output ---")
print(output)

# ── Cleanly close the SSH session ────────────────────────────────
connection.disconnect()
print("\nDisconnected.")
  

Expected Output

Connecting to NetsTuts-R1...
Connected! Prompt: NetsTuts-R1#

--- Raw Output ---
Interface              IP-Address      OK? Method Status                Protocol
GigabitEthernet0/0    192.168.10.1    YES NVRAM  up                    up
GigabitEthernet0/1    192.168.20.1    YES NVRAM  up                    up
GigabitEthernet0/2    unassigned      YES unset  administratively down down
Loopback0             10.255.255.1    YES NVRAM  up                    up
Serial0/1/0           203.0.113.1     YES NVRAM  up                    up

Disconnected.
  

5. Step 3 — Using the Context Manager (Best Practice)

The with statement (context manager) automatically calls disconnect() when the block exits — even if an exception is raised inside the block. This is the recommended pattern for all production Netmiko scripts:

# context_manager.py
# Recommended pattern: with statement ensures disconnect() always runs

from netmiko import ConnectHandler

device = {
    'device_type': 'cisco_ios',
    'host':        '192.168.10.1',
    'username':    'netauto',
    'password':    'AutoPass2026!',
}

# ── Context manager: disconnect() is called automatically ─────────
with ConnectHandler(**device) as conn:
    output = conn.send_command("show ip interface brief")
    print(output)
# disconnect() called automatically here — even if an exception occurred
  

6. Step 4 — Exception Handling

Network automation scripts run against live infrastructure. Devices can be unreachable, credentials can be wrong, or a device can crash mid-session. Proper exception handling prevents a single failed device from aborting a script that runs against dozens of devices:

# exception_handling.py
# Handle connection failures gracefully

from netmiko import ConnectHandler
from netmiko.exceptions import (
    NetmikoTimeoutException,       # TCP connection timed out (device unreachable)
    NetmikoAuthenticationException, # SSH auth failed (wrong username/password)
)

device = {
    'device_type': 'cisco_ios',
    'host':        '192.168.10.1',
    'username':    'netauto',
    'password':    'AutoPass2026!',
}

try:
    with ConnectHandler(**device) as conn:
        output = conn.send_command("show ip interface brief")
        print(output)

except NetmikoTimeoutException:
    # Device did not respond — unreachable IP, SSH port blocked, device down
    print("ERROR: Connection timed out. Is 192.168.10.1 reachable on TCP/22?")

except NetmikoAuthenticationException:
    # SSH connected but username/password was rejected by the router
    print("ERROR: Authentication failed. Check username and password.")

except Exception as e:
    # Catch-all for unexpected errors (SSH key mismatch, IOS bug, etc.)
    print(f"ERROR: Unexpected error — {type(e).__name__}: {e}")
  

7. Step 5 — Parsing Output Manually with Python

The raw output of show ip interface brief is a fixed-width text table. Python string methods and a regular expression can convert each line into a structured dictionary with named fields — making the data queryable and reusable:

# manual_parse.py
# Connect, retrieve output, parse it manually into a list of dicts

import re
from netmiko import ConnectHandler
from netmiko.exceptions import NetmikoTimeoutException, NetmikoAuthenticationException

device = {
    'device_type': 'cisco_ios',
    'host':        '192.168.10.1',
    'username':    'netauto',
    'password':    'AutoPass2026!',
}

def parse_ip_interface_brief(raw_output):
    """
    Parse 'show ip interface brief' text output into a list of dicts.

    Each dict has keys:
        interface, ip_address, ok, method, status, protocol

    Raw line example:
        GigabitEthernet0/0    192.168.10.1    YES NVRAM  up      up
    """
    interfaces = []

    # Regex: match each data line of show ip interface brief
    # Groups: (interface)(ip_address)(ok)(method)(status)(protocol)
    pattern = re.compile(
        r'^(\S+)\s+'          # Interface name (non-whitespace)
        r'(\S+)\s+'           # IP address or 'unassigned'
        r'(YES|NO)\s+'        # OK? field
        r'(\S+)\s+'           # Method (NVRAM, DHCP, manual, unset)
        r'([\w\s]+?)\s{2,}'  # Status (may contain spaces: 'administratively down')
        r'(\S+)$',           # Protocol (up or down)
        re.MULTILINE
    )

    for match in pattern.finditer(raw_output):
        interfaces.append({
            'interface':   match.group(1),
            'ip_address':  match.group(2),
            'ok':          match.group(3),
            'method':      match.group(4),
            'status':      match.group(5).strip(),
            'protocol':    match.group(6),
        })
    return interfaces

try:
    with ConnectHandler(**device) as conn:
        raw = conn.send_command("show ip interface brief")

    # Parse the raw string into structured data
    interfaces = parse_ip_interface_brief(raw)

    # ── Print structured output ───────────────────────────────────────
    print(f"{'Interface':<30} {'IP Address':<18} {'Status':<22} {'Protocol'}")
    print("-" * 78)
    for iface in interfaces:
        print(
            f"{iface['interface']:<30} "
            f"{iface['ip_address']:<18} "
            f"{iface['status']:<22} "
            f"{iface['protocol']}"
        )

    # ── Filter: show only interfaces that are DOWN ────────────────────
    print("\n--- Interfaces with issues (status or protocol down) ---")
    down_interfaces = [
        i for i in interfaces
        if i['status'] != 'up' or i['protocol'] != 'up'
    ]
    if down_interfaces:
        for i in down_interfaces:
            print(f"  {i['interface']}: {i['status']} / {i['protocol']}")
    else:
        print("  All interfaces up.")

except NetmikoTimeoutException:
    print("ERROR: Connection timed out.")
except NetmikoAuthenticationException:
    print("ERROR: Authentication failed.")
  

Script Output

Interface                      IP Address         Status                 Protocol
------------------------------------------------------------------------------
GigabitEthernet0/0             192.168.10.1       up                     up
GigabitEthernet0/1             192.168.20.1       up                     up
GigabitEthernet0/2             unassigned         administratively down  down
Loopback0                      10.255.255.1       up                     up
Serial0/1/0                    203.0.113.1        up                     up

--- Interfaces with issues (status or protocol down) ---
  GigabitEthernet0/2: administratively down / down
  

8. Step 6 — Automatic Parsing with TextFSM (Recommended)

Writing a custom regex for every show command is time-consuming and fragile. TextFSM is a template-based parsing engine. NTC-Templates is a community library of pre-written TextFSM templates for hundreds of standard Cisco (and other vendor) show commands — installed automatically with Netmiko. Setting use_textfsm=True in send_command() activates this automatic parsing:

# textfsm_parse.py
# Use use_textfsm=True to get structured data automatically

from netmiko import ConnectHandler
from netmiko.exceptions import NetmikoTimeoutException, NetmikoAuthenticationException

device = {
    'device_type': 'cisco_ios',
    'host':        '192.168.10.1',
    'username':    'netauto',
    'password':    'AutoPass2026!',
}

try:
    with ConnectHandler(**device) as conn:
        # use_textfsm=True: Netmiko selects the correct NTC-Templates
        # template for 'cisco_ios' + 'show ip interface brief'
        # and returns a list of dicts instead of a raw string
        interfaces = conn.send_command(
            "show ip interface brief",
            use_textfsm=True
        )

    # interfaces is now a list of dicts — no manual parsing needed
    print(f"Parsed {len(interfaces)} interfaces:\n")

    # ── Print all interfaces ──────────────────────────────────────────
    for iface in interfaces:
        print(f"  {iface['intf']:<28} {iface['ipaddr']:<18}"
              f" {iface['status']:<22} {iface['proto']}")

    # ── Filter: only UP/UP interfaces with a real IP ──────────────────
    print("\n--- Active interfaces with assigned IPs ---")
    active = [
        i for i in interfaces
        if i['status'] == 'up'
        and i['proto'] == 'up'
        and i['ipaddr'] != ''
    ]
    for i in active:
        print(f"  {i['intf']}: {i['ipaddr']}")

except NetmikoTimeoutException:
    print("ERROR: Connection timed out.")
except NetmikoAuthenticationException:
    print("ERROR: Authentication failed.")
  

TextFSM Output — Key Names for show ip interface brief

9. Step 7 — Run Against Multiple Devices

The real automation payoff comes when the same script runs against a list of devices. A function encapsulates the per-device logic; a loop drives it across the device inventory:

# multi_device.py
# Poll show ip interface brief from multiple routers

from netmiko import ConnectHandler
from netmiko.exceptions import NetmikoTimeoutException, NetmikoAuthenticationException

# ── Device inventory ──────────────────────────────────────────────
# In production: load this from a YAML/JSON file or a CMDB
DEVICES = [
    {'device_type': 'cisco_ios', 'host': '192.168.10.1', 'username': 'netauto', 'password': 'AutoPass2026!'},
    {'device_type': 'cisco_ios', 'host': '192.168.10.2', 'username': 'netauto', 'password': 'AutoPass2026!'},
    {'device_type': 'cisco_ios', 'host': '192.168.10.3', 'username': 'netauto', 'password': 'AutoPass2026!'},
]

def get_interfaces(device_dict):
    """Connect to one device, return parsed interface list or error string."""
    try:
        with ConnectHandler(**device_dict) as conn:
            return conn.send_command("show ip interface brief", use_textfsm=True)
    except NetmikoTimeoutException:
        return f"TIMEOUT: {device_dict['host']} unreachable"
    except NetmikoAuthenticationException:
        return f"AUTH FAIL: {device_dict['host']}"
    except Exception as e:
        return f"ERROR: {device_dict['host']} — {e}"

# ── Poll all devices sequentially ────────────────────────────────
for dev in DEVICES:
    print(f"\n{'='*60}")
    print(f"Device: {dev['host']}")
    print(f"{'='*60}")

    result = get_interfaces(dev)

    if isinstance(result, str):
        # Error string returned — print and move on
        print(result)
    else:
        # List of dicts returned — display interface table
        for iface in result:
            status_flag = "✓" if iface['status'] == 'up' else "✗"
            print(f"  {status_flag} {iface['intf']:<26} {iface['ipaddr']:<18} {iface['status']}/{iface['proto']}")
  

Multi-Device Output

============================================================
Device: 192.168.10.1
============================================================
  ✓ GigabitEthernet0/0       192.168.10.1       up/up
  ✓ GigabitEthernet0/1       192.168.20.1       up/up
  ✗ GigabitEthernet0/2       unassigned         administratively down/down
  ✓ Loopback0                10.255.255.1       up/up

============================================================
Device: 192.168.10.2
============================================================
  ✓ GigabitEthernet0/0       192.168.10.2       up/up
  ✓ GigabitEthernet0/1       192.168.30.1       up/up
  ✓ Loopback0                10.255.255.2       up/up

============================================================
Device: 192.168.10.3
============================================================
TIMEOUT: 192.168.10.3 unreachable
  

10. Step 8 — Save Output to File and Export to CSV

Structured data can be written to CSV for reporting in spreadsheet tools or to a JSON file for integration with other systems:

# save_output.py
# Export parsed interface data to CSV and JSON

import csv
import json
from datetime import datetime
from netmiko import ConnectHandler

device = {
    'device_type': 'cisco_ios',
    'host':        '192.168.10.1',
    'username':    'netauto',
    'password':    'AutoPass2026!',
}

with ConnectHandler(**device) as conn:
    hostname = conn.find_prompt().strip('#')
    interfaces = conn.send_command("show ip interface brief", use_textfsm=True)

timestamp = datetime.now().strftime("%Y%m%d_%H%M%S")

# ── Add device hostname and timestamp to each record ─────────────
for iface in interfaces:
    iface['device']    = hostname
    iface['timestamp'] = timestamp

# ── Write CSV ─────────────────────────────────────────────────────
csv_file = f"interfaces_{hostname}_{timestamp}.csv"
with open(csv_file, 'w', newline='') as f:
    writer = csv.DictWriter(f, fieldnames=['device','intf','ipaddr','status','proto','timestamp'])
    writer.writeheader()
    writer.writerows(interfaces)
print(f"CSV written: {csv_file}")

# ── Write JSON ────────────────────────────────────────────────────
json_file = f"interfaces_{hostname}_{timestamp}.json"
with open(json_file, 'w') as f:
    json.dump(interfaces, f, indent=2)
print(f"JSON written: {json_file}")
  

JSON Output Sample

[
  {
    "intf": "GigabitEthernet0/0",
    "ipaddr": "192.168.10.1",
    "status": "up",
    "proto": "up",
    "device": "NetsTuts-R1",
    "timestamp": "20260307_143022"
  },
  {
    "intf": "GigabitEthernet0/2",
    "ipaddr": "",
    "status": "administratively down",
    "proto": "down",
    "device": "NetsTuts-R1",
    "timestamp": "20260307_143022"
  }
]
  

11. Troubleshooting Netmiko Connection Issues

Enabling Netmiko Debug Logging

# debug_logging.py
# Enable Netmiko debug output to see exactly what is sent/received

import logging

# Set Netmiko logger to DEBUG — prints all SSH channel traffic
logging.basicConfig(level=logging.DEBUG)
logger = logging.getLogger('netmiko')

# Now run ConnectHandler — terminal shows every byte sent and received
from netmiko import ConnectHandler
conn = ConnectHandler(device_type='cisco_ios', host='192.168.10.1',
                          username='netauto', password='AutoPass2026!')
  

Key Points & Exam Tips

• Netmiko is a Python library built on Paramiko that abstracts SSH sessions to network devices. Its key value is vendor-specific driver support: it handles prompt detection, pagination (terminal length 0 is sent automatically), and enable mode for over 80 device types. Install with pip install netmiko.
• The device dictionary is the core pattern: all connection parameters (device_type, host, username, password) are passed as a dictionary to ConnectHandler(**device). This pattern makes it trivial to loop over a list of device dicts to poll multiple devices.
• The device_type parameter selects the correct driver. cisco_ios covers IOS and IOS-XE. Use cisco_nxos for Nexus, cisco_xr for IOS-XR, cisco_asa for ASA. The wrong device_type causes prompt detection failures or wrong paging-disable commands.
• send_command() sends a single command and returns the full output as a string once the device prompt reappears. It is the primary method for show commands. send_config_set() accepts a list of configuration commands, enters config mode, sends them, and exits — used for making changes.
• Always use the with context manager: with ConnectHandler(**device) as conn:. This guarantees disconnect() is called even if an exception occurs, preventing VTY line exhaustion on the device.
• use_textfsm=True in send_command() activates automatic parsing via NTC-Templates (installed with Netmiko). For standard show commands like show ip interface brief, it returns a list of dicts instead of a raw string — eliminating the need to write custom regex. If no template matches, it falls back to returning the raw string.
• The two critical Netmiko exceptions to handle are NetmikoTimeoutException (device unreachable) and NetmikoAuthenticationException (wrong credentials). Always wrap connection code in try/except to prevent one failed device from aborting a multi-device script.
• For privilege levels: configure the automation user with privilege 15 on the router to start directly at the # prompt. If the user has a lower privilege, add 'secret': 'enable_password' to the device dict and Netmiko will send enable automatically during connection setup. See Login Security & Brute-Force Protection to harden the VTY lines that Netmiko connects to.
• For large device inventories, parallel execution with concurrent.futures.ThreadPoolExecutor dramatically reduces total runtime. SSH connections are I/O-bound (waiting for the network), making them ideal candidates for threading — 50 parallel connections complete in roughly the same time as one sequential connection.
• For the CCNA automation objectives: understand the purpose of Netmiko (ConnectHandler for SSH connection), the device dictionary pattern, send_command() for show commands, send_config_set() for configuration, and the importance of disconnect() / context manager for clean session management. For additional Python networking scripts see Python Automation Scripts.

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