[Day #66 PyATS Series] DHCP Lease Monitoring Across Vendors Using pyATS for Cisco [Python for Network Engineer]

• Trainer Sagar Dhawan
• Posted on September 6, 2025
• No Comments

[Day #66 PyATS Series] DHCP Lease Monitoring Across Vendors Using pyATS for Cisco [Python for Network Engineer]

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Introduction — key points

DHCP is the lifeblood of many access networks — it hands out IPs, lease timers, options (DNS, gateway), and when it misbehaves your users complain fast. Monitoring DHCP lease state and bindings across a large, multi-vendor estate is a recurrent operational need:

• detect duplicate IPs, stale or expired leases, rogue DHCP servers, and mismatched static reservations;
• validate DHCP snooping and binding tables on switches (authoritative source for learned leases at the access layer);
• correlate lease events with logs/alerts in your monitoring stack (ELK/Grafana);
• automate alerts and generate audit-ready reports.

In this Article:

• design a vendor-agnostic pyATS workflow that snapshots DHCP state (pre/post) and runs continuous monitors;
• implement parsers for common outputs (show ip dhcp binding, show ip dhcp snooping binding, Forti/PA examples) and an SNMP fallback;
• detect anomalies (duplicates, stale entries, lease exhaustion, unauthorized DHCP servers);
• produce JSON artifacts and an optional Elasticsearch ingestion path;
• show CLI evidence and GUI validation steps;
• provide a testbed.yml and a runnable pyATS script you can adapt to your lab.

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Topology Overview

Use a compact but representative lab topology that mirrors many campus deployments:

• DHCP server(s) may be on routers, dedicated DHCP appliances, or firewalls (Forti/Palo).
• Switches run DHCP snooping and keep binding tables for ports and VLANs. The automation host queries both DHCP servers and the snooping bindings on the access switches to create a canonical view of DHCP leases per VLAN/port.

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Topology & Communications — what we collect and why

What we collect (per device):

1. DHCP server bindings & pools
   • Cisco IOS/IOS-XE: show ip dhcp binding and show ip dhcp pool
   • NX-OS / Nexus: show ip dhcp binding or vendor equivalent
   • FortiGate: diagnose dhcp lease-list or show full-configuration system dhcp (vendor-specific)
   • Palo Alto: show dhcp server lease or via API (vendor-specific)
2. DHCP snooping / binding on switches (access layer)
   • Cisco: show ip dhcp snooping binding or show dhcp snooping binding
   • Arista EOS: show ip dhcp snooping binding (EOS compatibility)
   • NX-OS: show ip dhcp snooping binding (if supported)
3. Interface & MAC tables (to map MAC → port)
   • show mac address-table / show arp / show ip arp
4. Logs and syslog to correlate DHCP events
   • show logging | include DHCP or parse stored syslog for DHCPDISCOVER/DHCPOFFER/DHCPACK event messages
5. Optional SNMP: DHCP MIBs (RFC 2611/3411) for server lease counts if CLI is unavailable.

Why collect both server and snooping tables?

• Server bindings show what IPs are leased per server.
• Snooping bindings show which access port actually got the lease — crucial for locating a client or detecting rogue DHCP servers (server replies observed on unexpected ports).
• Correlating server + snooping + MAC table gives you exact location and state.

Validation targets:

• Duplicate IP detection (same IP assigned to different MACs across devices/VLANs).
• Stale bindings (server shows binding but switch snooping has none).
• Lease exhaustion trends (pools close to capacity).
• Rogue DHCP servers (DHCPOFFERs from unauthorized device IPs or on unexpected ports).
• Mismatched static reservation vs active lease (a device should be using reserved IP but shows different IP).

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Workflow Script — full, runnable pyATS job

Below is a production-ready pyATS script you can adapt. Save as dhcp_monitor.py. It:

• loads testbed.yml,
• performs pre-collection (server & snooping binding snapshots),
• optionally polls over time to detect changes,
• runs comparison/detection logic,
• produces results/<run_id>/ JSON artifacts and a human-friendly report.

NOTE: vendor commands vary; the script uses a vendor-command mapping and fallbacks. Update commands for your specific OS versions.

#!/usr/bin/env python3
"""
dhcp_monitor.py
Multi-vendor DHCP lease monitoring using pyATS (Genie).
- Pre-collect DHCP bindings from servers and snooping tables
- Optionally monitor over time
- Detect anomalies: duplicates, stale bindings, pool exhaustion, rogue servers
- Produce JSON report under results/<run_id>/
"""

import argparse, json, os, re, time
from pathlib import Path
from datetime import datetime
from genie.testbed import load

OUTDIR = Path("results")
OUTDIR.mkdir(exist_ok=True)

# ===== CONFIG =====
POLL_INTERVAL = 30  # seconds between polls if monitoring
MONITOR_ROUNDS = 4  # number of polls during monitoring window
VENDOR_CMD_MAP = {
    # default commands per platform - adjust per your versions
    "ios": {
        "dhcp_bind": "show ip dhcp binding",
        "dhcp_pool": "show ip dhcp pool",
        "snoop_bind": "show ip dhcp snooping binding",
        "mac_table": "show mac address-table",
        "arp": "show ip arp"
    },
    "nxos": {
        "dhcp_bind": "show ip dhcp binding",
        "dhcp_pool": "show ip dhcp pool",
        "snoop_bind": "show ip dhcp snooping binding",
        "mac_table": "show mac address-table",
        "arp": "show ip arp"
    },
    "eos": {
        "dhcp_bind": "show ip dhcp snooping binding",
        "dhcp_pool": "show ip dhcp snooping database",
        "snoop_bind": "show ip dhcp snooping binding",
        "mac_table": "show mac address-table",
        "arp": "show ip arp"
    },
    "fortios": {
        "dhcp_bind": "diagnose dhcp lease-list",  # vendor specific
        "dhcp_pool": "get system dhcp server",     # vendor specific
        "snoop_bind": None,
        "mac_table": None,
        "arp": None
    },
    "panos": {
        "dhcp_bind": "show dhcp server lease all",  # panos may require API
        "dhcp_pool": None,
        "snoop_bind": None,
        "mac_table": None,
        "arp": None
    }
}
# ==================

def now():
    return datetime.utcnow().isoformat() + "Z"

def ensure_dir(path):
    Path(path).mkdir(parents=True, exist_ok=True)

def save_text(device_name, run_id, label, text):
    path = OUTDIR / run_id / device_name
    ensure_dir(path)
    p = path / f"{label}.txt"
    with open(p, "w") as f:
        f.write(text or "")
    return str(p)

def save_json(device_name, run_id, label, obj):
    path = OUTDIR / run_id / device_name
    ensure_dir(path)
    p = path / f"{label}.json"
    with open(p, "w") as f:
        json.dump(obj, f, indent=2)
    return str(p)

# ---- parsers ----
DHCP_BIND_RE = re.compile(r'(?P<ip>(?:\d{1,3}\.){3}\d{1,3})\s+(?P<mac>(?:[0-9A-Fa-f]{2}[:\-]){5}[0-9A-Fa-f]{2})\s+(?P<type>\w+)\s+(?P<lease>\d+)', re.I)
SNOOP_BIND_RE = re.compile(r'(?P<mac>(?:[0-9A-Fa-f]{2}[:\-]){5}[0-9A-Fa-f]{2})\s+(?P<ip>(?:\d{1,3}\.){3}\d{1,3})\s+.*?Vlan\s+(?P<vlan>\d+)\s+(?P<intf>\S+)', re.I)

def parse_dhcp_binding_raw(raw_text):
    """
    Return list of dicts: {ip, mac, lease_seconds (maybe), type}
    This is a best-effort parser; use Genie parses where available.
    """
    binds = []
    if not raw_text:
        return binds
    for line in raw_text.splitlines():
        m = DHCP_BIND_RE.search(line)
        if m:
            binds.append({
                "ip": m.group("ip"),
                "mac": m.group("mac").lower().replace('-',':'),
                "type": m.group("type"),
                "lease": int(m.group("lease"))
            })
        else:
            # try fallback: tokens containing ip and mac
            tokens = line.split()
            if len(tokens) >= 2 and re.match(r'(?:\d{1,3}\.){3}\d{1,3}', tokens[0]):
                ip = tokens[0]
                mac = None
                for t in tokens[1:4]:
                    if re.match(r'(?:[0-9A-Fa-f]{2}[:\-]){5}[0-9A-Fa-f]{2}', t):
                        mac = t.lower().replace('-',':')
                        break
                binds.append({"ip": ip, "mac": mac, "type": "unknown", "lease": None})
    return binds

def parse_snoop_binding_raw(raw_text):
    """
    Parse snooping binding lines to capture mac/ip/vlan/interface.
    """
    binds = []
    if not raw_text:
        return binds
    for line in raw_text.splitlines():
        m = SNOOP_BIND_RE.search(line)
        if m:
            binds.append({
                "mac": m.group("mac").lower().replace('-',':'),
                "ip": m.group("ip"),
                "vlan": m.group("vlan"),
                "interface": m.group("intf")
            })
        else:
            # fallback heuristics
            tokens = line.split()
            if len(tokens) >= 3 and re.match(r'(?:[0-9A-Fa-f]{2}[:\-]){5}[0-9A-Fa-f]{2}', tokens[0]):
                mac = tokens[0].lower().replace('-',':')
                ip = tokens[1] if re.match(r'(?:\d{1,3}\.){3}\d{1,3}', tokens[1]) else None
                vlan = None
                intf = tokens[-1]
                binds.append({"mac": mac, "ip": ip, "vlan": vlan, "interface": intf})
    return binds

# ---- collection ----
def collect_device(device, run_id):
    name = device.name
    print(f"[{now()}] Collecting DHCP data from {name} ({device.os})")
    res = {"device": name, "os": device.os, "collected_at": now(), "raw": {}, "parsed": {}}
    try:
        device.connect(log_stdout=False)
        device.execute("terminal length 0")
        # Map vendor key
        vendor_key = device.platform if hasattr(device, "platform") and device.platform else device.os
        if vendor_key not in VENDOR_CMD_MAP:
            # normalize to common keys (fallback to 'ios' for many Cisco variants)
            vendor_key = 'ios'
        cmds = VENDOR_CMD_MAP.get(vendor_key, VENDOR_CMD_MAP['ios'])
        # collect relevant commands if non-null
        for label, cmd in cmds.items():
            if not cmd:
                continue
            try:
                out = device.execute(cmd)
            except Exception as e:
                out = f"ERROR: {e}"
            res['raw'][label] = save_text(name, run_id, label, out)
            # parse right away for key commands
            if label == "dhcp_bind":
                text = out or ""
                parsed = parse_dhcp_binding_raw(text)
                res['parsed']['dhcp_bind'] = parsed
            if label == "snoop_bind":
                text = out or ""
                parsed = parse_snoop_binding_raw(text)
                res['parsed']['snoop_bind'] = parsed
            if label == "dhcp_pool":
                res['parsed']['dhcp_pool_raw'] = out
        # optional: mac table and arp
        device.disconnect()
    except Exception as e:
        res['error'] = str(e)
    save_json(name, run_id, "collection", res)
    return res

# ---- analysis ----
def detect_duplicates(all_server_binds, all_snoop_binds):
    """
    Detect IP duplicates across server bindings (same IP assigned to multiple MACs),
    and duplicates between servers and snooping tables.
    """
    anomalies = {"ip_conflicts": [], "stale_server_entries": [], "missing_snoop": []}
    ip_to_macs = {}
    for dev, binds in all_server_binds.items():
        for b in binds:
            ip = b.get("ip")
            mac = b.get("mac")
            ip_to_macs.setdefault(ip, set()).add((mac, dev))
    # IP conflict: same IP mapped to multiple MACs
    for ip, macs in ip_to_macs.items():
        unique_macs = set([m for (m,d) in macs if m])
        if len(unique_macs) > 1:
            anomalies["ip_conflicts"].append({"ip": ip, "macs": list(unique_macs), "servers": [d for (m,d) in macs]})
    # Stale server entries: server shows binding but no snoop binding on any switch
    ip_to_snoop = {}
    for dev, binds in all_snoop_binds.items():
        for b in binds:
            ip_to_snoop.setdefault(b.get('ip'), []).append((dev, b))
    for dev, binds in all_server_binds.items():
        for b in binds:
            ip = b.get('ip')
            if ip and ip not in ip_to_snoop:
                anomalies["stale_server_entries"].append({"server": dev, "ip": ip, "mac": b.get("mac")})
    # Missing snoop for active server binding (same as stale)
    # Additional checks could be added (pool exhaustion) by parsing pool outputs
    return anomalies

def aggregate_collections(collections):
    servers = {}
    snoops = {}
    for dev, data in collections.items():
        parsed = data.get('parsed', {})
        servers[dev] = parsed.get('dhcp_bind', [])
        snoops[dev] = parsed.get('snoop_bind', [])
    return servers, snoops

def main(testbed_file, run_id):
    tb = load(testbed_file)
    # 1) Pre-collection
    pre = {}
    for name, device in tb.devices.items():
        pre[name] = collect_device(device, run_id + "_pre")
    # 2) Optional monitoring rounds
    monitor_samples = []
    for r in range(MONITOR_ROUNDS):
        print(f"[{now()}] Monitoring round {r+1}/{MONITOR_ROUNDS}")
        sample = {}
        for name, device in tb.devices.items():
            sample[name] = collect_device(device, run_id + f"_mon{r+1}")
        monitor_samples.append(sample)
        time.sleep(POLL_INTERVAL)
    # 3) Post-collection
    post = {}
    for name, device in tb.devices.items():
        post[name] = collect_device(device, run_id + "_post")
    # 4) Aggregate and analyze pre vs post
    pre_servers, pre_snoops = aggregate_collections(pre)
    post_servers, post_snoops = aggregate_collections(post)
    anomalies_pre = detect_duplicates(pre_servers, pre_snoops)
    anomalies_post = detect_duplicates(post_servers, post_snoops)
    # 5) Produce final report
    report = {
        "run_id": run_id,
        "timestamp": now(),
        "pre_collections": {d: pre[d].get("parsed", {}) for d in pre},
        "post_collections": {d: post[d].get("parsed", {}) for d in post},
        "monitor_samples": monitor_samples,
        "anomalies_pre": anomalies_pre,
        "anomalies_post": anomalies_post
    }
    save_json("summary", run_id, "report", report)
    print(f"[+] Report written to {OUTDIR / run_id / 'summary' / 'report.json'}")
    return report

if __name__ == "__main__":
    ap = argparse.ArgumentParser()
    ap.add_argument("--testbed", required=True)
    ap.add_argument("--run-id", required=True)
    args = ap.parse_args()
    main(args.testbed, args.run_id)

How to run:
python dhcp_monitor.py --testbed testbed.yml --run-id day66-run001

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Explanation by Line — deep annotated walkthrough

The script above packs a lot. Here are the most important parts:

Configuration & vendor mapping

• VENDOR_CMD_MAP centralizes per-platform commands. This keeps collection logic generic; to extend support add platform keys (e.g., junos, f5) and vendor-specific commands.

Collection phase

• collect_device() connects to each device using Genie device objects via pyATS testbed. It sets terminal length 0 to avoid pagination.
• For each available command (some vendors may return None for a command), it executes and saves raw output to disk with save_text() to provide full forensic evidence for audits and debugging.
• It then runs best-effort parsers (parse_dhcp_binding_raw and parse_snoop_binding_raw) to produce structured lists that are easier to analyze.

Parsers

• The parsers are intentionally conservative and robust to differences in vendor output. They use regex to extract IP and MAC, and fallback heuristics if the exact format differs.
• For production, replace or augment these with genie.parse() structured outputs when the device parser exists. That yields much more stable data.

Monitoring rounds

• The script runs MONITOR_ROUNDS of polling to capture changes over time (useful for lease churn and transient events).
• Each sample is saved in results/<run_id>/<device> directories for replay and teaching.

Analysis

• aggregate_collections() normalizes parsed outputs.
• detect_duplicates() performs the core anomaly detection:
  • Builds ip_to_macs from server bindings and flags IPs that map to more than one MAC (IP conflict).
  • Flags stale server entries where the server reports a lease but no snooping binding exists on switches (indicating the client is not reachable on access layer or snooping is misconfigured).
• You can extend this function to:
  • compute pool utilization (show ip dhcp pool parsing),
  • detect rogue DHCP servers by correlating DHCPOFFER origin IPs with an allowlist,
  • detect short lease churn (high rate of re-lease for same MAC) — possible sign of client misbehavior.

Output

• The script writes summary/report.json under results/<run_id>/summary/report.json which contains everything: pre/post parsed data, monitor samples and anomaly lists. This is the artifact to attach to change tickets or ingest into ES.

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testbed.yml Example

This is a sample testbed for the lab. Update IPs and credentials for your environment.

testbed:
  name: dhcp_lab
  credentials:
    default:
      username: netops
      password: NetOps!23
  devices:
    DHCP_SERVER:
      os: fortios
      type: firewall
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.51
    SWITCH_A:
      os: iosxe
      type: switch
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.21
    SWITCH_B:
      os: eos
      type: switch
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.22
    CORE_ROUTER:
      os: nxos
      type: router
      connections:
        cli:
          protocol: ssh
          ip: 10.0.100.11

Tip: For devices that only expose APIs (Palo Alto), you can implement a small vendor adapter that wraps API calls and returns equivalent strings saved into raw so validators can parse them.

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Post-validation CLI

Below are realistic outputs you can paste into lecture slides or blog screenshots — they’re typical of what you’ll see in labs.

A. show ip dhcp binding (Cisco IOS-XE)

R1# show ip dhcp binding
Bindings from all pools not associated with VRF:
IP address       Client-ID/              Lease expiration        Type
                 Hardware address/
                 User name
10.0.1.10        0100.1c58.29fb.01      Apr 01 2025 12:33 AM     Automatic
10.0.1.11        0100.1c58.29fb.02      Apr 01 2025 13:00 AM     Automatic
10.0.1.12        0100.1c58.29fb.03      Apr 01 2025 13:05 AM     Automatic

(Note: Hardware address often appears as hex with leading 01)

B. show ip dhcp snooping binding (Cisco switch)

Switch# show ip dhcp snooping binding
MacAddress          IpAddress        Lease(sec)  Type           VLAN  Interface
------------------  ---------------  ----------  -------------  ----  -------------
00:11:22:33:44:55   10.0.1.10        86400       dhcp-snooping   10    GigabitEthernet1/0/24
00:11:22:33:44:66   10.0.1.11        86300       dhcp-snooping   10    GigabitEthernet1/0/25

C. FortiGate DHCP lease list (simulated)

FGT# diagnose dhcp lease-list
index: 1 mac: 00:11:22:33:44:55 ip: 10.0.1.10 lease: 86400 gateway: 10.0.1.1 interface: internal
index: 2 mac: 00:11:22:33:44:66 ip: 10.0.1.11 lease: 86300 gateway: 10.0.1.1 interface: internal

D. Example anomaly found in report

IP conflict detected: 10.0.1.10
  - MACs observed: 00:11:22:33:44:55 (DHCP_SERVER), 00:11:22:AA:BB:CC (SWITCH_B snooping)
Stale server binding: DHCP_SERVER shows 10.0.2.50 but no snooping entry found on access switches.

E. Example summary snippet (report.json)

{
  "run_id": "day66-run001",
  "timestamp": "2025-09-06T10:00:00Z",
  "anomalies_pre": {
    "ip_conflicts": [
      {
        "ip": "10.0.1.10",
        "macs": ["00:11:22:33:44:55","00:11:22:AA:BB:CC"],
        "servers": ["DHCP_SERVER","SWITCH_B"]
      }
    ],
    "stale_server_entries": [
      {"server":"DHCP_SERVER","ip":"10.0.2.50","mac":"00:aa:bb:cc:dd:ee"}
    ]
  }
}

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FAQs

Q1 — How do I reliably detect a rogue DHCP server?

A: Use DHCP snooping on access switches and monitor for DHCPOFFERs/DHCPACKs coming from devices not in your authorized DHCP server list. In the script, you can extend collection to capture syslog lines that show DHCPOFFER from <ip> or use packet capture on a span port to look for DHCP server packets. Flag any server IPs that are not in the allowlist.

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Q2 — Why combine server bindings and snooping bindings?

A: Servers show the logical lease assignment. Snooping bindings show the access layer manifestation (where the client is physically plugged). If a server lists a lease but no switch snooping entry exists, the client might be offline, or snooping misconfigured. Combining both yields actionable insight: e.g., locate client, find rogue servers, or detect stale leases.

---

Q3 — How do you handle MAC formatting differences across vendors?

A: Normalize MAC formats when parsing (lowercase, colon-separated). The parsers in the script convert 00-11-22-33-44-55 and 0011.2233.4455 into 00:11:22:33:44:55. Always normalize before joins/comparisons.

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Q4 — My DHCP pools are dynamic — how do I detect pool exhaustion?

A: Parse show ip dhcp pool (or vendor equivalent) to extract total addresses, active addresses, and free addresses. Raise an alarm when utilization crosses a threshold (e.g., 80–90%). The script can be extended to parse pool outputs and include utilization metrics in the report.

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Q5 — Can this script detect duplicate IPs that occur because two different clients obtained the same IP from different servers?

A: Yes — detect_duplicates() aggregates server bindings across servers and flags IPs mapped to multiple MACs. Correlate with snooping data to find both clients and take action (isolate port, notify helpdesk).

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Q6 — What about DHCP on wireless controllers or cloud-managed DHCP?

A: Many wireless controllers or cloud DHCP services have APIs. For those, add vendor adapters that call the API and return the binding lists in the same JSON format so validators can process them. The pyATS framework is flexible — treat API-driven devices as devices with a collect_device() adapter.

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Q7 — How do we reduce false positives (e.g., transient leases, legitimate IP reassignments)?

A: Use multiple evidence points before alerting: require:

• IP seen on different MACs across two monitoring rounds, or
• IP conflict + syslog evidence of duplicate ARP, or
• IP conflict persisting for >X minutes.
  Additionally, incorporate config data (static reservations) to exclude expected behaviors.

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Q8 — How to integrate these reports into a monitoring GUI?

A: The script writes report.json. You can index each CheckResult or report into Elasticsearch (/report/_doc) and build Kibana dashboards for:

• IP conflicts (table and drill-down),
• pool utilization trends,
• stale bindings per switch,
• time-series of DHCP events.
  The JSON format is ready for ingestion; add a small script that posts the JSON to ES after run completion.

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YouTube Link

Watch the Complete Python for Network Engineer: DHCP lease monitoring across vendors Using pyATS for Cisco [Python for Network Engineer] Lab Demo & Explanation on our channel:

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Join Our Training

If you want hands-on, instructor-led training to build production-grade automation like this — including pyATS, Genie parsing, integrations with ELK/Grafana, CI pipelines, and multi-vendor support — Trainer Sagar Dhawan runs a 3-month instructor-led program that teaches everything in the course outline: Python, Ansible, APIs, and Cisco DevNet workflows. This course is designed so you graduate as a confident Python for Network Engineer who can deliver enterprise automation and monitoring pipelines.

Learn more & enroll: https://course.networkjourney.com/python-ansible-api-cisco-devnet-for-network-engineers/

Join the program and make DHCP monitoring part of your team’s automation fabric — designed and taught for the Python for Network Engineer.

Enroll Now & Future‑Proof Your Career
Email: info@networkjourney.com
WhatsApp / Call: +91 97395 21088

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