---
title: "Intermittent 100G Link Drops: The Temperature Problem Nobody Talks About"
type: tutorial
target_audience: technical
score: 9/10
---

Intermittent link drops on 100G infrastructure have a specific failure signature that distinguishes them from every other cause: they correlate with time of day, not with traffic load, and they disappear entirely after a chassis reboot or when the data center HVAC cycles on. Most engineers, when they encounter this pattern, spend the first several hours pursuing the wrong suspects — firmware bugs, cable faults, module incompatibility — because the temperature relationship is not obvious until you overlay the link event log against the thermal data from the same time window. Once you see the correlation, it is unmistakable, and the subsequent repair is usually inexpensive. Getting to that correlation requires knowing what to look for.

QSFP28 modules operating at 100G use either a VCSEL array at 850 nm for SR4, or direct modulation DFB lasers at 1310 nm for LR4 and CWDM4. Both laser types have optical output power that is temperature-dependent. VCSELs typically have a negative temperature coefficient for threshold current and differential efficiency — as temperature increases, threshold current rises and differential efficiency (slope efficiency, measured in mW/mA) falls, meaning the laser requires more drive current to produce the same output power. DFB lasers used in LR4 and CWDM4 have an additional wavelength drift characteristic of approximately 0.1 nm per degree Celsius, which in a multiplexed CWDM4 system can cause channel crosstalk if the wavelength drifts sufficiently toward an adjacent CWDM grid slot.

The automatic power control circuit in the module compensates for temperature-induced output variation by adjusting TX bias current, which is why TX power in DOM typically reads stable even as cage temperature rises. The problem occurs when the cage temperature reaches the upper region of the QSFP28 operating range and the APC loop reaches its maximum bias current output. At that point, the loop can no longer maintain output power, TX power begins to drop below nominal, and if the optical path already had limited margin — a slightly long or attenuated fiber run, a marginally contaminated connector — the receiving module's RX power drops below its sensitivity floor. The link drops. Within a few minutes, the APC loop state is reset by the module's transient recovery behavior, or the ambient temperature cycles slightly downward, and the link comes back. The event log shows a single link drop of 45 seconds to several minutes. Repeat after a few hours.

The HVAC cycle correlation appears because most facility HVAC systems run on a setpoint control loop that allows the hot aisle temperature to rise 4 to 6°C above the setpoint before the cooling stage engages, then overshoots to 2 to 3°C below setpoint before the cooling stage cuts off. In a hot-aisle-contained pod with a setpoint of 35°C, the actual hot aisle temperature may cycle between 30°C and 41°C over a 20 to 40 minute period. A QSFP28 module in a rear-facing optical port on a chassis in the hot aisle sees cage temperatures that track this cycle with roughly a 5 to 10 minute thermal lag. If the module's marginal operating point is around 38°C cage temperature, it will fail intermittently twice per HVAC cycle and appear fine the rest of the time.

The DOM data that confirms this diagnosis is straightforward to extract if you know what to read. The temperature register in SFF-8636 reporting is the module die temperature, which is approximately 5 to 8°C above the cage inlet temperature for modules under full electrical load. A cage temperature of 38°C from the chassis thermal sensor corresponds to a module die temperature of roughly 43 to 46°C. The TX bias current register will be at or near its maximum alarm threshold — typically 15 to 17 mA for a 25G VCSEL lane — during the failure period. TX power, if the module is still in the APC recovery zone, may show a reduction of 0.5 to 1.5 dB below baseline. RX power on the far end will show a corresponding reduction. If you poll these registers at 60-second intervals over a 4-hour window that includes a suspected failure event, the temperature, bias current, and power traces will clearly show the thermal marginal behavior. The event log timestamp will fall within the period where temperature is at its peak.

The RX power alarm threshold is what most engineers watch, but the action threshold for thermal-marginal links should be the TX bias current high alarm on the transmitting module, not the RX power low alarm on the receiving module. The TX bias current approaches its maximum before TX power degrades to the point where RX power alarms trigger on the far end. Setting a custom high warning threshold on TX bias current at 80 percent of the alarm value — typically around 12 to 13 mA on a 25G VCSEL lane — gives approximately 30 to 60 minutes of advance warning before the link becomes marginal. This is a threshold adjustment that Flexoptix EEPROM programming can apply to deployed modules when the platform supports custom alarm threshold configuration through MDIO or I2C access.

The HVAC cycle test is the definitive confirmation of thermal root cause when the failure history is ambiguous. With access to the facility management system, read the return air temperature at the CRAC unit that serves the affected pod at one-minute intervals. Simultaneously poll module temperature, TX bias current, and RX power at the same interval. If the link events align with the hot peaks of the HVAC cycle — not with traffic peaks, not with spanning tree events, not with switch CPU load — thermal root cause is confirmed. This test takes four to six hours to produce unambiguous data, but it eliminates every other hypothesis simultaneously and directs remediation to exactly the right intervention.

Remediation options are ordered by cost and disruption. The least disruptive option is increasing the cooling setpoint margin so the hot aisle temperature does not reach the module's marginal operating point — but this requires coordination with facilities and may impact adjacent equipment. Moving the affected chassis to a lower-temperature position in the rack — modules run cooler in the front half of the rack compared to the rear — is often feasible without a maintenance window and can reduce cage temperature by 3 to 5°C on its own. Cleaning the chassis air filter, which on a Cisco Nexus 9300 or Arista 7280 can restrict airflow enough to raise cage temperature by 4 to 8°C when heavily loaded with particulate, is a maintenance action that frequently resolves thermal-marginal link problems at no cost. Module replacement is the last resort and is only warranted when the module's operating range is genuinely insufficient for the deployment environment, which in a correctly designed data center should be rare.

Night-time failure patterns that coincide with reduced occupancy, lower IT load, and HVAC setback cycles are a distinct thermal failure mode. Some facilities programs reduce cooling output during off-peak hours based on occupancy or IT load projections, and the modules that were operating with a few degrees of thermal margin during business hours become marginally operational at 3 AM when the cooling capacity is reduced. The on-call engineer who gets paged at 2:47 AM for a flapping 100G link in an otherwise stable environment, who cycles the interface and watches it recover, who closes the ticket as "interface reset," has just papered over a thermal problem that will recur on the next HVAC setback cycle. The correct action is to poll DOM temperature data before clearing the alert and correlate with the facility thermal schedule.
