Measured Downtime During Inline Insertion of a Transparent Firewall
This test measures the interruption window observed during inline insertion of a transparent-mode firewall.
Test Objective
To evaluate the impact of physically inserting a transparent firewall into an active network path, focusing on real-world deployment conditions.
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.
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.
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.
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.
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.