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Proven Troubleshooting and Recovery Cases in Enterprise Networks
Proven Network Troubleshooting Cases
These network troubleshooting cases were resolved in enterprise production environments.
Cisco SD-WAN
We resolved operational limitations in Cisco SD-WAN environments by introducing automation.
Tasks difficult to perform through the GUI were implemented using Python.
Legacy TeraMacro procedures were translated into Python using generative AI, including automated configuration backup operations.
To prevent operational mistakes, a safety mechanism was implemented:
if the expected management IP address does not exist in the configuration, the script automatically stops.
All intellectual property must remain with the client.
Therefore, we use client-owned generative-AI environments when generating scripts.
Any modern tool must be usable by anyone.
If only specialists can operate it, it has limited value.
TeraMacro training costs are extremely low.People simply buy used routers on Yahoo Auctions (typically only a few thousand yen).
During pre-deployment validation, we discovered that the legacy BGP command “allow-as in” cannot be implemented in SD-WAN.
We resolved this using redistribution and route filtering.
IOS-XE 9200/9300 Switching
We resolved an issue where the command
“no spanning-tree vlan xx”
could not be applied.
The issue was solved using BPDU filter and BPDU guard.
Other members had been unable to resolve it before our intervention.
AWS
When web filtering was enabled, uploads failed with a probability of roughly 19 out of 20 attempts.
Root cause:
- Global IP changed mid-session due to virtual server relocation
- Non-DNS-based algorithm
- Packet fragmentation preventing Layer-7 inspection
This was confirmed through packet capture analysis.
VPN / IPsec
We identified incorrect hardware selection in a failed data-leak-prevention deployment.
We resolved a billing-related issue in an on-demand VPN circuit where packets continued arriving after communication completion due to IPsec confirmation behavior.
We also resolved:
- QoS not functioning with IPsec
- MTU issues caused by key-length changes
- Customer concerns about unencrypted voice packets (disproved through waveform analysis)
Layer 7-2 (Transparent IPS/WAF)
In transparent IPS/WAF environments, HSRP hello frames and R-STP BPDU frames did not pass by default on certain platforms.
Impact:
- HSRP Active-Active state
I “came up with” the idea on the spot to roll back using an RJ-45 J-J connector. - Up to 5 minutes of network outage
This issue had remained unresolved for three years.
We also discovered in advance that Auto-MDI becomes disabled when a transparent IPS loses power, which can cause link failure with fixed-speed devices.
Layer 4-3 (Load Balancers / Firewalls)
We discovered source-port exhaustion and TIME_WAIT reuse issues when SNAT was enabled on load balancers.
We also resolved:
- RedHat memory exhaustion caused by RST-terminated health checks
- Embryonic timeout issues
- TraceRoute being SNATed by default
- Firewall uRPF alerts triggered by TraceRoute
All confirmed via packet capture.
Issue: Juniper SRX repeatedly rebooting
On a Juniper SRX, after initiating a reboot via the serial console and command line, all activity stopped for more than ten minutes.
I suspected it had frozen and pressed keys on the PC keyboard, but there was no response.
During that period, the characters I typed (including Enter) accumulated in the PC’s keyboard buffer.
After a while, the log shows that for a brief moment it displayed: “press any key to reboot.”
Palo Alto (FW) — Failover Delay Analysis
The cause of the unexpected time consumed during failover was identified from the physical topology diagram.
The HA link between the primary and secondary units had been connected through a switch.
MAC flapping was recorded in the switch logs, indicating that this switch-mediated HA connection was the root cause of the failover delay.
Key Finding
The HA link must be directly connected between the primary and secondary units.
Introducing a switch in between can lead to MAC flapping and increased failover time.
Layer 3 Routing
Troubleshooting: QoS, NAT, Stateful NAT, PIM Multicast, and HSRP
We discovered incorrect QoS + NAT implementation described in Cisco documentation.
ACLs referencing IP addresses did not produce expected results.
Using port-based ACLs resolved the issue.
Wireshark graphs changed from a sawtooth pattern to a straight line, proving QoS effectiveness.
We also:
- Identified QoS misconfiguration with priority queue
- Predicted CPU overload during NAT migration
- Discovered stateful NAT left unconfigured for five years
- Confirmed PIM multicast and HSRP interoperability
Layer 2 Switching
We proved that some switches configured for untagged VLANs forward all tagged frames regardless of VLAN ID.
We also:
- Prevented STP root-bridge takeover during switch addition
- Resolved multicast MAC conflicts between IGMP and BPDU
Layer 1 / Wi-Fi / Bluetooth
We resolved Wi-Fi multicast performance degradation caused by lack of Layer-1 ACK.
On Cisco WLC, converting multicast to unicast resolved the issue.
Bluetooth Noise Investigation
Using the Ukrainian-made spectrum analyzer IT24, we verified and demonstrated that no significant noise was present within the Bluetooth frequency band.
Crosstalk Issue in AI-Based Noise Cancellation
Using a phase-inversion analog noise canceller, we suppressed background voices located behind the telephone operator, addressing the crosstalk issue.
Radio Environment Verification Using a Spectrum Analyzer
By continuously recording the display and control screen of a Chinese-made spectrum analyzer (RF Explorer) over an extended period, we demonstrated that no interference caused by electromagnetic waves was present in the VIC, aeronautical radio, or weather satellite frequency bands.
Electromagnetic Leakage Measurement Around Power Systems Using Fluke
We were asked to assess the risk of potential TEMPEST-type attacks. By leveraging the credibility and measurement capability of Fluke instruments, we demonstrated that no meaningful electromagnetic leakage was occurring from the power systems.
Gray-Zone Optimization
By configuring the wireless access point to use right-hand circular polarization, we reduced Wi-Fi interference and channel congestion.
Exoneration of Suspected Interference Points Using a Noise Generator
Using a Noise Generator manufactured by Japan’s CosmoWave, we demonstrated that the cause of the VoIP communication issues was not electromagnetic interference.
Verifying Server Power Supply Redundancy Using Power Line Communication (PLC)
We measure and confirm whether primary and secondary power redundancy is properly established by using PLC (Power Line Communication) as a diagnostic tool.
Work in Progress: 10 GHz Band & Future Wi-Fi Measurement Prototype
- Early prototype for a 10 GHz-band measurement platform
Development is underway for a measurement device targeting the 10 GHz range, with a roadmap toward future Wi-Fi analysis and verification tools. The goal is to establish a practical, field-deployable measurement environment rather than a purely laboratory-grade instrument. - Hardware status
- LNB (Low-Noise Block converter) already procured and validated for integration.
- Bias-T circuit currently under soldering optimization and impedance tuning for stable DC feed and RF isolation.
- Purpose of this prototype
This pre-production model is intended to support high-frequency evaluation, signal-path verification, and future expansion toward professional Wi-Fi measurement workflows. It is being built with a vendor-neutral design philosophy and a focus on real-world troubleshooting scenarios. - Next steps
- Finalize Bias-T assembly and stability testing.
- Integrate LNB with measurement chain.
- Validate repeatability and noise characteristics.
- Prepare for extension into Wi-Fi measurement use cases.
Status: Ongoing engineering work. Detailed specifications will be published after verification.
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Related Architecture Solutions
Typical network architecture solutions designed and implemented by GIT. These patterns are derived from real enterprise environments and long-term operational experience.
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