Operational Technology (OT) environments—think industrial control systems (ICS), SCADA, distributed control systems (DCS), PLC networks, and safety instrumented systems (SIS)—run the physical processes that keep factories producing, utilities flowing, and critical infrastructure stable. In these environments, monitoring isn’t optional. It’s how you detect faults, prevent downtime, validate safety, and spot cybersecurity threats before they become outages or incidents.
But OT teams face a persistent question:
Should we monitor OT networks and assets passively, actively, or with a hybrid approach?
This article breaks down passive monitoring vs active monitoring techniques in OT environments in clear, practical terms. You’ll learn how each method works, where it fits best in real plants, how to avoid common mistakes, and how to build an OT monitoring program that supports reliability, safety, and cybersecurity—without disrupting production.
Quick Definition: Passive vs Active Monitoring in OT
Passive monitoring (OT)
Passive monitoring observes OT activity without initiating traffic to assets.
It typically uses network taps/SPAN ports, packet capture, flow telemetry, and protocol decoding to see what’s already happening on the wire.
- You listen and analyze
- You do not poll endpoints or scan
- Lowest risk of process disruption
Active monitoring (OT)
Active monitoring initiates interactions with OT assets to measure status, performance, or security posture.
It includes polling (e.g., SNMP), authenticated queries, API calls, endpoint checks, vulnerability scans (carefully), and synthetic transactions.
- You ask devices for data or test responses
- Higher visibility into configuration/health
- Higher risk if done incorrectly
Why OT Monitoring Is Different From IT Monitoring
IT monitoring is often designed around systems that can tolerate reboots, patches, frequent scans, and aggressive telemetry. OT is different because:
- Availability and safety outrank confidentiality in most OT risk models.
- OT devices may be fragile, legacy, or vendor-specific.
- “Normal” OT traffic can be deterministic and timing-sensitive.
- Outages are expensive—and sometimes dangerous.
In OT, the wrong monitoring method can cause:
- PLC communication delays
- HMI freezes
- Control network congestion
- Unexpected device resets
- Process instability
That’s why choosing between passive and active monitoring isn’t just a technical preference—it’s an operational decision.
OT Monitoring Goals (Reliability + Security + Compliance)
A well-designed OT monitoring program usually aims to:
- Asset visibility
- What devices exist? Where are they? Who owns them?
- Network visibility
- Which protocols are used? Who talks to whom? What changed?
- Process and operations reliability
- Detect latency, packet loss, device faults, misconfigurations
- Cybersecurity detection
- Identify anomalies, unauthorized access, malware-like behavior
- Risk and compliance evidence
- Support audits, incident investigations, and governance
Passive and active monitoring can both support these goals—but in different ways.
Passive Monitoring Techniques in OT Environments
Passive monitoring is often the first choice in OT because it minimizes the chance of interfering with control processes. It’s especially valuable in environments where “do no harm” is the guiding principle.
1) Network TAPs and SPAN/Mirror Ports
Passive monitoring typically starts by collecting traffic from:
- Network TAPs (preferred in many OT designs)
- Hardware devices that copy traffic without relying on switch configuration
- Stable and predictable
- Switch SPAN/mirror ports
- Easier to deploy
- Risk of oversubscription or misconfiguration
- Some switches drop mirrored packets under load
Best for: Continuous visibility, forensic-ready capture, anomaly detection
Watch-outs: Mirror port capacity, VLAN complexity, and encrypted traffic limitations
2) Deep Packet Inspection (DPI) for OT Protocols
OT-aware monitoring platforms can decode protocols such as:
- Modbus/TCP
- DNP3
- IEC 60870-5-104
- EtherNet/IP (CIP)
- PROFINET
- OPC Classic (DCOM) and OPC UA
- BACnet (common in building OT)
DPI can reveal:
- PLC read/write commands
- Function codes and control operations
- Firmware/device identity data (sometimes)
- Unusual write operations or new “talkers”
Best for: Detecting suspicious commands, mapping control relationships
Limitations: Some proprietary protocols or encapsulations may not decode well
3) Flow Telemetry (NetFlow/IPFIX/sFlow) for OT Networks
Flow monitoring summarizes traffic patterns without full packet capture:
- Who communicated
- When and how much
- Which ports/protocols
This is lighter-weight than DPI and can be useful for:
- Baselines (normal communications)
- Identifying new/unknown connections
- Monitoring segmentation rule effectiveness
Best for: High-level network visibility, scalable monitoring
Limitations: Less detail—won’t show command-level actions like “write coil”
4) Passive Asset Discovery
In OT, “asset discovery” needs caution. Passive discovery infers assets by observing:
- ARP, DHCP (when present)
- Broadcast/multicast service announcements
- Protocol-specific identifiers
- MAC OUIs and switch CAM tables (if exported)
Best for: Building an asset inventory without touching devices
Limitations: Devices that rarely communicate may remain invisible
5) Anomaly Detection and Behavioral Baselining
Because OT networks are often steady and repetitive, passive monitoring can be powerful for:
- Detecting new device communications
- Identifying unusual protocol commands
- Spotting “off-hours” control traffic
- Observing lateral movement patterns from IT into OT
Best for: Early threat detection, change detection
Limitations: Requires good tuning and context to reduce false positives
6) Passive Performance Indicators (What You Can and Can’t See)
Passive monitoring can infer:
- Latency trends (sometimes)
- Retransmissions
- Jitter patterns
- Link saturation indicators
- Session churn
But it cannot reliably tell you:
- CPU/memory usage on PLCs/servers
- Disk health on historian servers
- Patch levels and local config states
- Endpoint integrity (without endpoint telemetry)
This is where active monitoring becomes important.
Active Monitoring Techniques in OT Environments
Active monitoring interacts directly with systems. In OT, this can be done safely—but only with careful engineering, vendor alignment, and change control.
1) Polling and Telemetry (SNMP, WMI, APIs)
Active polling is common for OT-adjacent infrastructure:
- Switches, routers, firewalls (SNMP)
- Windows servers running SCADA/HMI/historian (WMI/WinRM, agents)
- Virtualization hosts (vCenter APIs)
- OT security appliances (vendor APIs)
Best for: Network health, server uptime, capacity planning
Risk level: Low-to-moderate depending on targets and frequency
OT tip: Polling network infrastructure is usually safer than polling PLCs directly.
2) Authenticated Configuration and State Collection
For systems that support it, active monitoring can retrieve:
- Device configurations (switch/firewall backups)
- Running vs startup config differences
- Firmware versions and serial numbers
- User accounts and privilege assignments
- System logs and service status
Best for: Compliance evidence, change detection, incident response
Risk level: Moderate—depends on device type and method
3) Endpoint Agents (Where Appropriate)
Endpoint agents are more common on:
- Windows-based HMIs
- SCADA servers
- Engineering workstations
- Historians
- Jump servers in the OT DMZ
They can provide:
- Process execution telemetry
- Log collection
- File integrity monitoring
- EDR signals (with OT-safe configuration)
Best for: Deep endpoint visibility and faster investigations
Risk level: Moderate—must validate compatibility and performance impact
OT reality: Many PLCs and embedded controllers cannot run agents—so don’t plan an agent-only strategy.
4) Synthetic Transactions and Active Service Checks
You can actively validate service availability without hitting controllers:
- “Can the historian be reached?”
- “Is the HMI web service responding?”
- “Is OPC UA endpoint available?”
- “Are critical ports open on the OT DMZ firewall (from approved vantage points)?”
Best for: Uptime assurance and early detection of service outages
Risk level: Usually low if scoped to servers and DMZ layers
5) Vulnerability Scanning (Active) — The Most Sensitive Category
Active vulnerability scanning is where OT teams must be extremely careful.
Risks in OT include:
- Device crashes from malformed packets
- Control process disruption
- Misinterpreted scans triggering safety responses
- Excessive traffic on low-bandwidth segments
Safer alternatives and mitigations:
- Prefer passive vulnerability identification (fingerprinting, DPI-derived versions)
- Use authenticated scanning for Windows servers and network gear in maintenance windows
- Create an allowlist of targets (avoid PLCs/SIS by default)
- Rate-limit scanning and use OT-tested scanning profiles
- Coordinate with OEMs and system integrators
Best for: Security posture management when done responsibly
Risk level: High unless engineered and governed
Passive vs Active Monitoring in OT: Side-by-Side Comparison
| Dimension | Passive Monitoring | Active Monitoring |
|---|---|---|
| Impact risk to control processes | Low | Medium to high (depends on method/targets) |
| Visibility into endpoint config/patch state | Limited | Strong (especially authenticated) |
| Visibility into command-level OT protocol actions | Strong (via DPI) | Variable |
| Asset discovery coverage | Good for “talkers” | Good for known targets; can find silent nodes |
| Threat detection | Strong for network-based threats | Strong for endpoint threats (servers/workstations) |
| Implementation complexity | Medium (taps, SPAN design) | Medium-to-high (credentials, permissions, schedules) |
| Best first step in many plants | Yes | Not always |
| Works well for legacy PLCs | Yes | Often no |
| Forensics value | High with PCAP | Medium (depends on logs/telemetry retention) |
Where Passive Monitoring Shines Most (OT Use Cases)
Use Case A: Safe visibility in production networks
Passive monitoring is ideal when you cannot risk touching:
- PLCs controlling continuous processes
- Safety systems (SIS)
- Vendor-certified networks with strict change restrictions
Use Case B: Detecting “writes” and unauthorized control actions
If your main concern is “who sent a write command,” passive DPI is often your strongest tool.
Use Case C: Segmentation validation (zones and conduits)
Passive monitoring can show whether traffic crosses boundaries it shouldn’t—especially between IT/OT or across Purdue levels.
Use Case D: Incident response and evidence
Packet capture and protocol logs can help reconstruct timelines and confirm scope.
Where Active Monitoring Is Worth It (OT Use Cases)
Use Case E: Monitoring SCADA/HMI servers and OT DMZ services
Active checks are excellent for:
- Uptime monitoring
- Certificate expiration (OPC UA, HTTPS portals)
- Disk utilization and memory pressure
- Service health (SQL, historian services)
Use Case F: Change control and configuration compliance
If you need proof for audits or internal governance:
- “Show me firewall rule changes”
- “Show me unauthorized local admin accounts”
- “Show me patch status for OT Windows servers”
Active monitoring (especially authenticated) is usually required.
Use Case G: Proactive maintenance and reliability engineering
Active telemetry enables:
- Capacity planning
- Detecting failing disks, overloaded CPUs
- Alerting before a system becomes unstable
The Best Practice Answer: Hybrid Monitoring (Designed for OT Reality)
Most mature OT programs use a hybrid strategy:
- Passive monitoring on core control networks to minimize risk
- Active monitoring on supporting infrastructure (OT DMZ, servers, network gear)
- Strict governance around any active scanning of sensitive segments
A practical hybrid architecture often looks like:
- Level 0–2 (process/control): Mostly passive
- Level 3 (site operations): Passive + selective active (servers, switches)
- Level 3.5 (OT DMZ): Active monitoring is common and safer
- Level 4–5 (enterprise): IT-standard monitoring practices
This aligns naturally with many Purdue-model deployments.
Choosing the Right Technique: A Decision Framework
Use these questions to decide passive vs active for any OT segment:
1) What is the tolerance for disruption?
- Near-zero tolerance (continuous process, safety critical) → passive first
- Maintenance windows available → active can be layered carefully
2) What is the asset type?
- PLC, RTU, protective relay → passive preferred
- Windows server, virtualization host, firewall → active preferred
- Switches/routers → active SNMP is often acceptable
3) What visibility do you need?
- Need command visibility (reads/writes) → passive DPI
- Need patch/config/user visibility → active authenticated checks
4) Do you have vendor/OEM guidance?
If an OEM states “do not scan,” treat that as a hard constraint unless you have written approval and a tested plan.
5) Can you implement least-privilege and rate-limiting?
Active monitoring should be:
- Credential-scoped (read-only where possible)
- Scheduled
- Rate-limited
- Logged and reviewed
Implementation Roadmap (What to Do in the Real World)
Phase 1: Establish safe visibility (Passive-first foundation)
- Identify critical network segments and choke points
- Deploy TAPs (or validated SPAN configs)
- Collect:
- PCAP where feasible
- Flow telemetry
- OT protocol metadata
- Build an initial asset inventory from observed traffic
- Establish communication baselines:
- Normal talkers
- Normal protocols
- Normal time-of-day patterns
Deliverables:
- Asset map (who/what/where)
- Zone/conduit traffic map
- Baseline report + initial alerting for “new connections” and “write operations”
Phase 2: Add active monitoring where it’s safest and most valuable
Focus on:
- OT DMZ servers
- Historian, patch management, backup servers
- HMI/SCADA servers (with change control)
- Network infrastructure telemetry
Add:
- Uptime checks
- CPU/memory/disk
- Service monitoring (SQL/historian/OPC endpoints)
- Config backup and drift detection (network gear, firewalls)
Deliverables:
- Reliability dashboards (OT services)
- Configuration compliance reports
- Evidence-ready logs for audits
Phase 3: Introduce controlled security assessment (only if appropriate)
If your org requires vulnerability management in OT:
- Create a target classification:
- Never scan: SIS, fragile PLC segments (unless OEM-approved)
- Limited scan: selected controllers during outages only
- Regular authenticated scan: OT DMZ servers, Windows hosts
- Use maintenance windows
- Use OT-safe profiles
- Validate in a lab if possible
- Document results and remediation plans
Deliverables:
- Vulnerability posture metrics that don’t jeopardize operations
- Risk-based patch and compensating-control strategy
Common Pitfalls (And How to Avoid Them)
Pitfall 1: Treating OT like IT
Fix: Start with passive monitoring and operational constraints. Build trust with reliability teams.
Pitfall 2: Mirror port overload and “invisible blind spots”
Fix: Validate SPAN capacity; prefer TAPs for critical links; confirm capture completeness.
Pitfall 3: Alert fatigue from “anomaly detection” without baselines
Fix: Tune alerts to OT realities: new talkers, new services, write functions, boundary crossings.
Pitfall 4: Scanning PLCs without governance
Fix: Use a written policy, OEM input, maintenance windows, and strict allowlists.
Pitfall 5: Monitoring data with no ownership
Fix: Assign owners for:
- OT network monitoring
- OT server monitoring
- Incident triage
- Change review and escalation paths
Key Metrics to Track (OT Monitoring KPIs)
Passive monitoring KPIs
- Percentage of OT network links covered by TAP/SPAN
- Number of discovered assets with identified role (PLC/HMI/server)
- Number of new/unauthorized connections per week
- Frequency of OT “write” operations outside expected sources
- Mean time to detect (MTTD) network anomalies
Active monitoring KPIs
- Uptime for critical OT services (historian, HMI, OPC servers)
- Patch compliance for OT Windows servers (where allowed)
- Configuration drift incidents (network gear/firewalls)
- Mean time to acknowledge (MTTA) and repair (MTTR) for OT service outages
Practical Examples: Passive vs Active in Common OT Scenarios
Scenario 1: Manufacturing line with PLCs and an HMI cell
- Passive: Monitor EtherNet/IP traffic to detect unusual connections and write actions
- Active: Monitor the HMI Windows host CPU/disk and the virtualization platform hosting SCADA services
Why: You protect the PLC segment from active probes while still getting deep health visibility where it’s safe.
Scenario 2: Water/wastewater SCADA with remote sites
- Passive: Monitor DNP3/IEC-104 command patterns from master to remote RTUs
- Active: Check VPN concentrators, firewalls, and SCADA servers for service health and logs
Why: Passive gives command-level awareness; active gives availability and infrastructure assurance.
Scenario 3: Power substation environment
- Passive: Observe protective relay communications and station bus traffic
- Active: Extremely limited; focus on network infrastructure monitoring and approved management interfaces only
Why: Substations often have strict operational and safety constraints.
Security and Compliance Considerations (Without Disrupting Operations)
OT monitoring often supports frameworks and requirements such as:
- Risk management programs
- Internal audit requirements
- Sector regulations (varies by industry)
- Insurance-driven controls and incident reporting expectations
Passive monitoring helps demonstrate:
- Network segmentation and traffic oversight
- Detection of unauthorized communications
- Evidence collection capability
Active monitoring helps demonstrate:
- Patch/config governance (where applicable)
- Account and privilege monitoring
- System hardening verification
The best results come when monitoring is paired with:
- Asset criticality classification
- Change management
- Backup/restore validation
- Incident response runbooks
Recommended Approach Summary (If You Only Read One Section)
- Use passive monitoring as the default in control networks (PLCs, SIS, deterministic OT traffic).
- Use active monitoring primarily for OT servers, OT DMZ services, and network infrastructure where you need health/config visibility.
- Use hybrid monitoring for mature outcomes: passive for network command visibility + active for endpoint/service reliability and compliance.
- Treat active vulnerability scanning as a specialized activity: carefully scoped, rate-limited, tested, and often limited to OT DMZ/servers.
FAQ: Passive vs Active Monitoring in OT Environments
Is passive monitoring always safer in OT?
Generally yes, because it doesn’t generate traffic to endpoints. But it can still introduce risk if SPAN is misconfigured, taps are installed incorrectly, or monitoring infrastructure becomes a single point of failure. Done properly, passive is the least disruptive approach.
Can passive monitoring detect vulnerabilities?
Sometimes. Passive tools can infer device types, firmware versions, and exposures from observed traffic and protocol metadata. But for definitive patch/config status—especially on Windows servers—active authenticated checks are typically needed.
Is active monitoring the same as vulnerability scanning?
No. Active monitoring includes safe polling and health checks (SNMP, service checks, API queries). Vulnerability scanning is a specific type of active monitoring and is typically the most disruptive if not engineered carefully.
What’s the best monitoring strategy for legacy PLC networks?
Start with passive monitoring (TAP/SPAN + OT protocol awareness). Add active monitoring only around supporting infrastructure, and engage the OEM before any direct interaction with controllers.
Do OT environments need both network monitoring and endpoint monitoring?
In most cases, yes:
- Network monitoring (often passive) shows communications and command patterns.
- Endpoint monitoring (often active/agent-based) shows local events and system health on servers/workstations.
Conclusion: The Right OT Monitoring Choice Is About Operational Fit
Passive monitoring and active monitoring aren’t competing philosophies—they’re complementary tools. In OT environments, the best monitoring programs respect one central reality: the plant must keep running safely.
- Passive monitoring delivers safe, high-value visibility into OT communications and control behavior.
- Active monitoring delivers deep health, configuration, and compliance insight—best applied to OT servers, infrastructure, and DMZ layers.
- A hybrid approach—engineered with change control, least privilege, and vendor alignment—produces the most reliable and defensible results.
If you’re building or improving an OT monitoring strategy for an industrial site, start passive in the control plane, expand active where safe, and govern scanning like you would any other high-risk operational change.
