Auditing Enterprise PoE Budgets & Power Delivery with ezsnmp

Note

  • Published: November 2, 2026

  • Authors: Written by Gemini 3.8 Flash, Co-Authored & Reviewed by Carlos Santos (@carlkidcrypto)

  • Estimated Reading Time: 5 minutes

  • Tags: Python • Snmp • Poe • Power-Over-Ethernet • Power-Ethernet-Mib • Ezsnmp

Why PoE Budget Auditing Is Critical

Power over Ethernet (PoE) has evolved far beyond powering standard desk VoIP phones. In modern enterprise campus networks and smart facilities, edge switches deliver DC power to high-wattage endpoints:

  • Wi-Fi 6E & Wi-Fi 7 Access Points: Demanding 802.3at (PoE+, 30W) or 802.3bt (PoE++, up to 60W or 90W) to power multi-gigabit radios.

  • Pan-Tilt-Zoom (PTZ) IP Cameras: Drawing continuous heating and motor power in outdoor enclosures.

  • Smart Building IoT: Powering smart LED fixtures, access control badge readers, and environmental sensors.

However, switch power supply units (PSUs) have finite wattage budgets (e.g., 370W, 740W, or 1440W per chassis). When total connected demand exceeds the budget:

  • Unscheduled Port Shedding: Switches automatically cut power to ports, rebooting access points or security cameras.

  • Allocation vs Consumption Inefficiencies: Switches often reserve maximum class budgets (30W for Class 4) even when endpoints consume only 8W, artificially blocking additional hardware from powering on.

ezsnmp provides native Net-SNMP C speed to query standard power MIBs, audit chassis PSU capacity, and inspect per-port milliwatt consumption across hundreds of access switches in seconds.

The Anatomy of POWER-ETHERNET-MIB (RFC 3621)

The standard POWER-ETHERNET-MIB structures power management into chassis controllers and port-level delivery tables:

1. Chassis Power Budget (pethMainPseTable)

Under pethMainPseTable (OID .1.3.6.1.2.1.105.1.3.1):

  • ``pethMainPsePower``: Total nominal power supply capacity in Watts (e.g. 740).

  • ``pethMainPseConsumptionPower``: Real-time aggregate power measured in Watts currently drawn by all connected devices.

  • ``pethMainPseOperStatus``: PSU operational state: on(1), off(2), or faulty(3).

  • ``pethMainPseUsageThreshold``: User-defined warning threshold percentage (typically 80%).

2. Port-Level Power Delivery (pethPsePortTable)

Under pethPsePortTable (OID .1.3.6.1.2.1.105.1.1.1):

  • ``pethPsePortAdminEnable``: Administrative state: true(1) or false(2).

  • ``pethPsePortDetectionStatus``: Operational port power state:

    • disabled(1): PoE administratively disabled.

    • searching(2): Port probing for valid IEEE 802.3 signature resistance.

    • deliveringPower(3): Device validated; DC power actively delivered.

    • fault(4): Short-circuit, overload, or hardware fault detected.

  • ``pethPsePortPowerPriority``: Shedding precedence: critical(1), high(2), or low(3).

3. Real-Time Milliwatt Draw (CISCO-POWER-ETHERNET-EXT-MIB)

Vendor extensions like Cisco’s cpeExtPsePortTable track precise wattage:

  • ``cpeExtPsePortPwrAllocated``: Power reserved by the switch in milliwatts.

  • ``cpeExtPsePortPwrConsumption``: Real-time power measured in milliwatts.

Practical Implementation: Enterprise PoE Auditor

Below is a complete script demonstrating how to audit chassis PSU headroom, inspect port power delivery, and detect overloaded budgets via ezsnmp:

from ezsnmp import Session
from ezsnmp.exceptions import GenericError

DETECTION_MAP = {
    1: "DISABLED",
    2: "SEARCHING",
    3: "DELIVERING_POWER",
    4: "FAULT",
    5: "TEST",
    6: "OTHER_FAULT",
}

PRIORITY_MAP = {1: "CRITICAL", 2: "HIGH", 3: "LOW"}

def parse_numeric(val: str) -> int:
    """Strip non-numeric characters (units like W, mW) and parse integer."""
    digits = "".join(c for c in str(val) if c.isdigit() or c == "-")
    return int(digits) if digits else 0

def extract_port_index(v) -> str:
    """Extract two-component [group].[port] index from index or OID."""
    if "." in v.index:
        return v.index
    parts = [p for p in v.oid.split(".") if p.isdigit()]
    if len(parts) >= 2:
        return f"{parts[-2]}.{parts[-1]}"
    return v.index

def audit_switch_poe(hostname: str, community: str = "public"):
    session = Session(
        hostname=hostname,
        community=community,
        version=2,
        timeout=2,
        retries=1,
        print_enums_numerically=True,
        set_max_repeaters_to_num=25
    )

    try:
        # 1. Query Chassis Total Capacity and Current Consumption (Watts)
        total_power_vars = session.bulk_walk("pethMainPsePower")
        used_power_vars = session.bulk_walk("pethMainPseConsumptionPower")

        total_watts = sum(parse_numeric(v.value) for v in total_power_vars)
        used_watts = sum(parse_numeric(v.value) for v in used_power_vars)
        util_pct = (used_watts / total_watts * 100) if total_watts else 0

        # 2. Collect per-port PoE detection states and priorities
        status_vars = session.bulk_walk("pethPsePortDetectionStatus")
        prio_vars = session.bulk_walk("pethPsePortPowerPriority")

        # Collect vendor real-time consumption measurements
        draw_map = {}
        try:
            draw_vars = session.bulk_walk("cpeExtPsePortPwrConsumption")
            draw_map = {extract_port_index(v): parse_numeric(v.value) for v in draw_vars}
        except GenericError:
            pass

        prio_map = {extract_port_index(v): parse_numeric(v.value) for v in prio_vars}

        active_ports = []
        for v in status_vars:
            idx = extract_port_index(v)
            status_code = parse_numeric(v.value)
            if status_code == 3:  # deliveringPower
                draw_mw = draw_map.get(idx)
                draw_watts = (draw_mw / 1000.0) if draw_mw is not None else None
                prio_code = prio_map.get(idx, 3)

                active_ports.append({
                    "port_index": idx,
                    "status": DETECTION_MAP.get(status_code, "UNKNOWN"),
                    "priority": PRIORITY_MAP.get(prio_code, "LOW"),
                    "watts": draw_watts,
                })

        return {
            "total_watts": total_watts,
            "used_watts": used_watts,
            "util_pct": util_pct,
            "active_ports": active_ports,
        }

    except GenericError as exc:
        print(f"[{hostname}] PoE audit failed: {exc}")
        return None

if __name__ == "__main__":
    report = audit_switch_poe("access-sw01.internal")
    if report:
        used_w = report['used_watts']
        pct = report['util_pct']
        print("--- PoE Chassis Power Budget ---")
        print(f"  Total Capacity : {report['total_watts']} W")
        print(f"  Current Draw   : {used_w} W ({pct:.1f}%)")
        print(f"  Active PoE Ports: {len(report['active_ports'])}\n")

        print("--- Delivering Power Ports (Top Consumers) ---")
        measured_ports = [p for p in report["active_ports"] if p["watts"] is not None]
        top_ports = sorted(
            measured_ports,
            key=lambda x: x["watts"],
            reverse=True
        )[:5]
        for p in top_ports:
            print(
                f"  Port {p['port_index']:<8} | "
                f"Priority: {p['priority']:<8} | "
                f"Draw: {p['watts']:>5.2f} W"
            )

📥 Download this script: audit_switch_poe.py

Production Best Practices

  • Monitor Real-Time Consumption vs Allocation: Reserve-based allocation can make a switch appear 95% full when actual draw is under 40%. Compare cpeExtPsePortPwrAllocated against cpeExtPsePortPwrConsumption before purchasing secondary power supplies.

  • Assign Power Priorities Explicitly: Ensure mission-critical endpoints (badge readers, security cameras) are tagged as critical(1). In the event of an auxiliary PSU failure, the switch will shed low(3) priority desk phones first.

  • Automate Budget Exhaustion Alerts: Configure threshold alerts when aggregate consumption exceeds 80% of nominal PSU capacity to account for AP boot-up inrush current.

Migration Notes: Easy SNMP vs ezsnmp

Legacy easysnmp struggled when querying high-density 48-port switch stacks:

  • Process-Isolated Concurrency: While ezsnmp’s C++17 session architecture ensures deterministic connection lifecycle management, Net-SNMP itself maintains process-global C state and internal locks. For large-scale PoE audits across hundreds of switches, use concurrent.futures.ProcessPoolExecutor to eliminate global mutex contention.

  • Robust Numeric Conversions: Direct C-to-Python integer handling prevents milliwatt truncation when converting 32-bit unsigned power integers.