VPC Flow Analysis Reference

Use VPC Flow Logs to analyze traffic patterns, volume, and latency.

🤖 Agent / Gemini CLI Instructions (MCP)

Agents should use Cloud Logging MCP for exploratory analysis or BigQuery MCP for high-volume trends. Fallback to the CLI if the MCP tools are not available.

Exploratory Analysis: Typically involves looking at individual log entries or a small set of logs to understand specific events, debug issues, or investigate anomalies. This often requires filtering and examining the full details of log records.
High-Volume Trends: Focuses on aggregating large datasets of logs over time to identify patterns, measure traffic volumes, analyze latency distributions, or find "top talkers." This usually involves SQL queries to summarize data rather than inspecting individual logs.

1. View Logs (Cloud Logging MCP)

Tool: list_log_entries

Filter: ALWAYS search for both VPC flow log sources:

(logName:"projects/{project_id}/logs/compute.googleapis.com%2Fvpc_flows" OR
logName:"projects/{project_id}/logs/networkmanagement.googleapis.com%2Fvpc_flows")
resource.type="gce_subnetwork"

2. Aggregate Trends (BigQuery MCP)

Tool: execute_sql

SQL Pattern:

SELECT timestamp,
JSON_VALUE(jsonPayload.connection.src_ip) AS src_ip,
JSON_VALUE(jsonPayload.connection.dest_ip) AS dest_ip,
CAST(JSON_VALUE(jsonPayload.bytes_sent) AS INT64) AS bytes_sent FROM
`{project_id}.{dataset_id}._AllLogs` WHERE log_name IN (
'projects/{project_id}/logs/compute.googleapis.com%2Fvpc_flows',
'projects/{project_id}/logs/networkmanagement.googleapis.com%2Fvpc_flows' ) AND
timestamp >= TIMESTAMP_SUB(CURRENT_TIMESTAMP(), INTERVAL 1 HOUR) ORDER BY
timestamp DESC LIMIT 10

3. CLI Fallback

If MCP tools are unavailable, use the following gcloud and bq commands:

View Logs (gcloud)

gcloud logging read '(logName:"projects/{project_id}/logs/compute.googleapis.com%2Fvpc_flows" OR logName:"projects/{project_id}/logs/networkmanagement.googleapis.com%2Fvpc_flows") AND resource.type="gce_subnetwork"' --project {project_id} --limit 10 --format json --quiet

Aggregate Trends (bq)

bq query --use_legacy_sql=false --project_id {project_id} '
SELECT
  timestamp,
  JSON_VALUE(json_payload.connection.src_ip) AS src_ip,
  JSON_VALUE(json_payload.connection.dest_ip) AS dest_ip,
  CAST(JSON_VALUE(json_payload.bytes_sent) AS INT64) AS bytes_sent
FROM `{project_id}.{dataset_id}._AllLogs`
WHERE
  log_name IN (
    "projects/{project_id}/logs/compute.googleapis.com%2Fvpc_flows",
    "projects/{project_id}/logs/networkmanagement.googleapis.com%2Fvpc_flows"
  )
  AND timestamp >= TIMESTAMP_SUB(CURRENT_TIMESTAMP(), INTERVAL 1 HOUR)
ORDER BY timestamp DESC
LIMIT 10
'

Flow Analyzer (Visual Analysis)

For visual traffic analysis and identifying "top talkers," use Flow Analyzer. It allows you to:

Visualize traffic flows between regions, VPCs, and instances.
Filter by source or destination dimensions.
Identify high-bandwidth or high-latency connections.

Generic BigQuery Guidelines

Schema Verification: Before executing a BigQuery query, if you are uncertain of the casing (for example, jsonPayload versus json_payload), you MUST run bq show --schema <source>.
Latency Aggregation: The primary field for RTT analysis in VPC Flow logs is json_payload.round_trip_time.median_msec. This field offers sub-millisecond precision and covers both TCP and Falcon traffic. Filter by reporter (SRC or DEST) to avoid double-counting traffic volume.

For TCP-only traffic, you can also use json_payload.rtt_msec, which provides RTT in whole milliseconds. While less precise and with narrower coverage than round_trip_time.median_msec, it can be aggregated as follows:
```
SELECT
  AVG(json_payload.rtt_msec) AS average_rtt_msec,
  MAX(json_payload.rtt_msec) AS max_rtt_msec
FROM ...
```

Key Fields

src_ip / dest_ip: Source and destination IP addresses.
bytes_sent / packets_sent: Volume of traffic.
round_trip_time.median_msec: The primary field for RTT analysis. This double field provides the median latency in milliseconds with sub-millisecond precision. It is populated for both TCP and Falcon traffic.
rtt_msec: An int64 field representing Round-trip time in whole milliseconds. Populated only for TCP traffic. Generally, round_trip_time.median_msec is preferred due to higher precision and broader coverage.
reporter: Usually src or dest indicating which side logged the flow.

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