Designing a secure and scalable OCI network is only half the job. In real-world enterprise environments, networks evolve continuously—routes change, security rules get updated, hybrid links fluctuate, and workloads scale dynamically. Without strong observability and troubleshooting capabilities, even the best-designed architectures can fail operationally.
Oracle Cloud Infrastructure (OCI) provides native tools that give deep visibility into network behavior, traffic flows, routing decisions, and connectivity paths.
This blog focuses on how to monitor, analyze, and troubleshoot OCI networks effectively using Flow Logs, Logging Analytics, Path Analyzer, and operational diagnostic tools.
1. VCN Flow Logs – Foundational Network Visibility
What VCN Flow Logs Are
VCN Flow Logs capture metadata about IP traffic flowing to and from a VNIC or subnet within a VCN. They do not capture packet payloads; instead, they record key attributes such as source and destination IP, ports, protocol, action (ALLOW/DENY), bytes transferred, and timestamps.
Flow Logs provide a factual record of how network security and routing decisions are applied in practice—not how they are intended to work.
Why Flow Logs Are Critical
Flow Logs form the ground truth for network troubleshooting. When traffic fails, they help answer questions like:
- Did the packet reach the subnet or VNIC?
- Was it allowed or denied?
- Which port or protocol was involved?
- Was the issue security-related or routing-related?
Without Flow Logs, architects often rely on assumptions.
How to Enable VCN Flow Logs
- Navigate to Networking → Virtual Cloud Networks.
- Select the Subnet and select monitoring tab
- Enable Flow Logs.
- Select or create a Log Group.
- Save the configuration.
Like shown in the above diagram, we can enable the flow logs for the subnet, it will show what traffic is coming to the selected subnet, whether its accepted or rejected with source and destination IP details.
You can also select the log retention period as it takes the cost for the object storage space. By default its 30 days as shown in the above diagram.
Common Use Cases
- Troubleshooting “instance not reachable” issues.
- Verifying NSG or Security List behavior.
- Auditing hybrid traffic entering via DRG.
- Identifying suspicious inbound connection attempts.
2. Logging Analytics – Making Sense of Flow Logs at Scale
What Logging Analytics Is
Logging Analytics is OCI’s centralized log analytics and observability platform. It ingests logs (including VCN Flow Logs), indexes them, applies machine learning, and presents insights through dashboards, queries, and alerts.
While Flow Logs capture raw data, Logging Analytics turns data into insights.
Why Logging Analytics Matters
In large environments, Flow Logs can generate millions of records. Logging Analytics enables:
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Pattern detection (normal vs abnormal traffic)
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Traffic trend analysis over time
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Rapid filtering of denied or suspicious flows
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Correlation across network, compute, and application logs
It significantly reduces mean time to resolution (MTTR).
How to Enable Logging Analytics
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Navigate to Observability & Management → Logging Analytics.
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Enable the Logging Analytics service for the tenancy.
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Grant required IAM policies for log ingestion.
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In OCI Logging, configure Flow Logs to be forwarded to Logging Analytics.
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Use built-in parsers and dashboards to visualize traffic.
you can enable the logging analytics from the home screen as highlighted in below screen.
Common Use Cases
Identifying spikes in denied traffic after rule changes.
Monitoring hybrid connectivity stability.
Investigating security incidents using historical logs.
Creating alerts for repeated denied connections.
3. Path Analyzer – Understanding Routing and Security Decisions
What Path Analyzer Does
Path Analyzer simulates the network path traffic would take between two endpoints in OCI. It evaluates route tables, gateways, NSGs, security lists, and
DRG attachments to determine whether traffic is allowed and how it is routed.
This is not packet capture—it is deterministic path simulation based on configuration.
Why Path Analyzer Is Essential
In complex environments—hub-spoke VCNs, hybrid connectivity, multi-region architectures—routing behavior can be difficult to predict. Path Analyzer eliminates guesswork by showing:
- Which route table is selected
- Which gateway is used
- Whether security rules allow traffic
- Where traffic would be dropped
How to Use Path Analyzer
- Navigate to Networking → Network Path Analyzer.
- Define the Source (instance, VNIC, or IP).
- Define the Destination (instance, CIDR, on-prem IP).
- Select protocol and port.
- Run the analysis to view the simulated path and results.
you can do bidirectional as well as unidirectional analysis also depending on the issue.
Once you have given all the source, destination, port, protocol and direction details then run the analysis.
Common Use Cases
- Debugging hub-spoke connectivity failures.
- Validating DRG routing logic.
- Troubleshooting asymmetric routing.
- Confirming hybrid traffic paths.
- Diagnosing backend reachability behind load balancers.
4. Common OCI Networking Troubleshooting Scenarios
Instance Not Reachable
This typically results from:
- Missing or incorrect route table entries.
- NSG or Security List blocking traffic.
- Incorrect gateway (IGW vs NAT).
- OS-level firewall restrictions.
Flow Logs identify denies, while Path Analyzer reveals routing issues.
Hybrid Connectivity Issues
Common causes include:
- IPSec tunnel down or BGP not established.
- Overlapping CIDR blocks.
- Incorrect DRG route table association.
- MTU mismatches causing silent packet drops.
- Missing return routes on on-prem routers.
Hybrid issues often require validating both OCI and customer-edge configuration.
Routing Conflicts
These occur due to:
- Overlapping VCN CIDRs.
- Incorrect DRG attachment route priorities.
- Multiple gateways competing for the same prefix.
Path Analyzer is the fastest way to isolate such issues.
Conclusion
Monitoring and troubleshooting are not optional in cloud networking—they are foundational. OCI provides a robust set of native tools that allow architects to observe
real traffic behavior, simulate routing decisions, and diagnose issues across VCNs, DRGs, and hybrid environments. By combining Flow Logs, Logging Analytics, Path Analyzer, and operational tools like CLI and traceroute, teams can significantly reduce outages, improve reliability, and maintain confidence in complex OCI network architectures.
This concludes the OCI Networking Series, where we explored networking from fundamentals to advanced monitoring and troubleshooting. Thank you for following along — see you soon in the next one...!!!!
Happy Networking. :)
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