Measured Downtime During Inline Insertion of a Transparent Firewall

This test measures the interruption window observed during inline insertion of a transparent-mode firewall.

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Test Objective

To evaluate the impact of physically inserting a transparent firewall into an active network path, focusing on real-world deployment conditions.

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Test Environment

• Firewall: FortiGate (transparent mode)
• Upstream device: Cisco CBS250-8T-D-JP (default settings)
• Client: Windows PC
• Topology: PC → FortiGate → Router → Internet

The firewall was fully booted and operational before insertion.

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Test Method

A continuous ICMP echo request was sent to a public endpoint.

ping 8.8.8.8 -t | ForEach-Object { "{0:HH:mm:ss.fff} {1}" -f (Get-Date), $_ }

During the test, the WAN-side cable of the firewall was removed and immediately reinserted, simulating a real inline deployment operation.

The test was performed multiple times under identical conditions. The maximum observed interruption window was used for evaluation, to reflect a conservative estimate suitable for real-world deployment planning.

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Observed Result

21:34:45.787 8.8.8.8 からの応答: バイト数 =32 時間 =3ms TTL=117
21:34:50.571 要求がタイムアウトしました。
21:34:55.575 要求がタイムアウトしました。
21:34:56.585 8.8.8.8 からの応答: バイト数 =32 時間 =4ms TTL=117

The last successful reply was recorded at 21:34:45.787, and successful replies resumed at 21:34:56.585.

This indicates an observed interruption window of approximately 10.8 seconds.

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Interpretation: Alignment with Theoretical L1 Recovery

The observed interruption window of 10.8 seconds is highly consistent with standard enterprise-grade network hardware behavior, rather than an arbitrary delay.

Theoretical Basis: Most managed switches and firewalls utilize a 10-second L1 Keepalive/Link-up timer (often influenced by carrier delay settings) by default.

Validation of Predictability: Our measurement of 10.8 seconds (the interval between the last successful ICMP reply and the first recovered reply) confirms that the downtime is strictly dictated by physical layer link-state detection.

Engineering Conclusion: This result demonstrates that inserting a transparent-mode firewall does not trigger unpredictable software-level re-convergence or routing instability. The downtime is transparent, predictable, and scientifically grounded within standard L1 recovery intervals.

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Operational Considerations

In real deployments, sufficient maintenance time must be secured in advance, as even short interruptions can impact active sessions and services.

This measurement prioritizes worst-case behavior over average performance, to support safe deployment planning.

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Related Evidence

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