Meeting SI.L2-3.14.6 under NIST SP 800-171 Rev.2 / CMMC 2.0 Level 2 requires a documented, operational plan to monitor network traffic so that suspicious activity affecting Controlled Unclassified Information (CUI) and other sensitive assets is detected, investigated, and responded to—this post gives a practical, step-by-step blueprint for small businesses (and compliance teams) to build an actionable monitoring plan aligned to the Compliance Framework.
What the requirement expects and how to scope it
At a high level the control expects continuous capability to observe network flows and packets at key boundaries and internal choke points, with alerts, logging, and retention sufficient to support detection and forensic investigation. For the Compliance Framework: document which network segments carry CUI (or could impact CUI), define monitoring objectives (detection of exfiltration, lateral movement, C2, anomalous protocols), and produce the artifacts auditors expect—network diagrams, sensor placement map, alerting rules, retention schedules, and SOPs for analyst triage.
Step-by-step implementation plan
Start with an inventory: list routers, firewalls, switches with mirror/span/TAP capability, cloud VPC flow logs, VPN concentrators, and wireless controllers. Classify traffic zones (internet edge, DMZ, CUI VLANs, server farm, guest). For each zone pick monitoring points: north–south at internet gateways, east–west within the datacenter (SPAN or TAP), and on cloud public subnets using flow logs (AWS VPC Flow Logs, Azure NSG Flow Logs). Document these points in the plan with IP ranges and physical/virtual device IDs.
Selecting tools and deployment patterns
Choose a layered tooling approach: flow collection (NetFlow/sFlow/IPFIX) for high-level baselining and spike detection; IDS/IPS (Suricata/Snort or managed service) for signature-based alerts; network security monitoring (NSM) with Zeek (Bro) for protocol parsing and rich metadata; and a SIEM (Elastic, Splunk, QRadar) or MSSP portal to correlate alerts and retain logs. For small businesses, a viable stack is: cloud VPC Flow Logs + Zeek on a small Linux host for CUI VLANs + Suricata on internet edge + ELK stack or managed Elastic Cloud. Use TAPs or switch SPAN on critical links; avoid SPAN for high-loss links—use TAPs if possible.
Specific technical configuration examples
Example configurations you should include in the plan: (1) NetFlow exporter on your core router sending UDP 2055 to a collector with 1-minute export interval; (2) Zeek capture on a mirrored link with rotation: 1GB PCAP trigger and 30-day rolling PCAP retention in compressed format, with triggered longer captures on indicator match; (3) Suricata rule to alert on large DNS TXT responses (possible data exfil): alert dns any any -> any any (msg:"Large DNS TXT - potential exfil"; dns_query; content:!"."; threshold: type both, track by_src, count 5, seconds 60; sid:1000001; rev:1;). Capture device timestamps with NTP and include timezone/UTC in logs.
Operational practices and evidence for auditors
Operationalize the plan with daily/weekly checks and evidence items: a monitoring runbook (who escalates, how to open an incident ticket), a log retention policy (e.g., flow metadata 365 days, SIEM normalized events 180 days, full PCAP 30 days except on incident), proof of sensor configuration (configs and checksums), and periodic validation tests (simulated exfiltration like a controlled large DNS TXT or HTTPS upload to a test sink). Maintain change history for rule updates and record false-positive tuning with timestamps and approver IDs—auditors want to see an iterative program, not a single snapshot.
Small business scenario (practical example)
Consider a 50-person engineering firm that handles CUI in design files. Budget limits mean they cannot run a large commercial SIEM. A practical plan: enable AWS VPC Flow Logs for cloud workloads, deploy a single Zeek appliance on-prem to monitor the office-to-data center link, run Suricata on the internet firewall, and ship logs to Elastic Cloud (hosted). Use a managed detection service or a part-time MSSP for alert triage. Document the architecture, sample alerts (exfiltration to foreign IPs, large SMB file transfers to external hosts), and SOPs for isolating affected VLANs—this meets the spirit of SI.L2-3.14.6 while being cost-aware.
Risks of not implementing the requirement
Without an actionable network traffic monitoring plan you risk undetected data exfiltration, delayed incident response, extended dwell time for adversaries, and inability to provide forensic evidence after a breach. From a compliance perspective, failure to implement this control can result in audit findings, potential contract termination with federal customers, and liability if CUI is compromised. Operationally, lack of monitoring increases remediation costs and reputational damage.
Compliance tips and best practices
Practical tips: (1) Map your monitoring plan directly to SI.L2-3.14.6 in a one-page traceability matrix; (2) prioritize CUI paths first—monitor where CUI lives and where it traverses; (3) keep rule changes minimal and documented; (4) automate health checks for collectors and set alerts for dropped packets or missed export rates; (5) use retention justifications tied to your risk assessment; and (6) run quarterly tabletop exercises to validate detection-to-response timelines. For evidence, export dashboard screenshots, rule files, and incident tickets with timestamps and analyst notes.
Summary: Building an actionable network traffic monitoring plan to satisfy NIST SP 800-171 Rev.2 / CMMC 2.0 SI.L2-3.14.6 requires scoping CUI paths, selecting layered tooling (flows, IDS, NSM, SIEM), placing sensors at key boundaries, documenting configurations and SOPs, validating detection with tests, and retaining the right artifacts for audits—implement these practical steps and small-business patterns to reduce risk and demonstrate compliance.
