1. Summary
This article describes an incident where a faulty fan module in a
Cisco Nexus leaf switch caused physical-layer instability, leading to
intermittent connectivity loss for critical cluster nodes. The issue resulted
in a cluster-wide access outage. This article documents the symptoms, root
cause analysis, and corrective actions to prevent a recurrence.
Key Findings:
Recurrent hardware failures of Fan
module 1 (Fan1) were directly correlated with physical interface flaps on the
connected ports (Ethernet1/27 and Ethernet1/43). The issue was
environmental/hardware-related and not due to software, configuration, or
traffic congestion.
2. Symptoms
During the incident window, the following symptoms were observed:
3. Environment
Device:
Cisco
Nexus Leaf Switch (Hostname: T10PHGXINLFSWXX)
Connected
Assets:
Two critical cluster nodes.
4. Root Cause Analysis
The root cause was a hardware failure of Fan Module 1
(Fan1) .
Primary
Cause:
The faulty fan module created an unstable hardware
environment, potentially through thermal stress or electrical issues. This
instability directly impacted the physical-layer integrity of nearby
components or backplane connections, triggering link flaps on Ethernet1/27 and Ethernet1/43.
Contributing
Factor:
The fan's status oscillated between 'OK' and 'FAIL',
indicating an intermittent hardware fault that was difficult to diagnose
without close log correlation.
Exclusions:
A thorough analysis of show
tech-support logs and buffer statistics confirmed the issue was not caused
by:
Micro-burst
traffic or congestion.
Buffer
exhaustion or ECN marking.
Recent
configuration or software changes.
Issues
on other interfaces or the management plane.
5. Scope of Impact
Direct
Impact:
Loss of network connectivity for two critical nodes
connected to the affected interfaces.
Business
Impact:
Cluster-wide service disruption, operational risk to
dependent applications. No data loss was reported.
6. Resolution and Corrective Actions
The following steps were recommended by Cisco TAC to resolve the
immediate issue and prevent its recurrence.
Immediate Remediation Steps
Reseat/Replace
Cables:
Reseat the fiber or copper cables connected to Ethernet1/27 and Ethernet1/43 to
ensure a secure connection.
Reseat/Replace
Optics:
Reseat or replace the transceiver modules (SFPs) on
both ends of the affected links.
Check
Remote End:
Inspect the interfaces and optics
on the remote devices connected to these ports for any errors or
instability.
Permanent Fix
Replace
Faulty Hardware:
Replace Fan Module 1 (Fan1) via
RMA (Return Material Authorization). This is the definitive fix
to eliminate the source of hardware instability.
Post-Replacement
Monitoring:
After the fan replacement, closely
monitor the following for at least 24-48 hours:
System
logs for any FAN_BAD or other environmental alerts.
Interface
status and error counters for Ethernet1/27 and Ethernet1/43.
Overall
switch health and environmental data (temperature, voltage).
7. Preventive Measures
To prevent similar incidents in the future, the following
recommendations should be implemented:
Proactive
Environmental Monitoring:
Enable and actively monitor
SNMP traps and system logs for hardware alerts (like FAN_BAD, PFM_ALERT,
temperature warnings). Integrate these alerts into a network monitoring
system (e.g., SolarWinds, PRTG, Nagios).
Immediate
Alert Escalation:
Treat repeated hardware
environmental alerts as high-priority incidents. Do not assume a temporary
fan recovery means the issue is resolved.
Scheduled
Physical Inspections:
Periodically inspect and clean
optical cables and transceivers on all critical links as part of routine
maintenance.
Design
for Redundancy:
Where possible, architect critical
node connections with redundancy (e.g., using NIC teaming, vPC, or
dual-homed connections) to mitigate the impact of a single point of
failure like a switch port or module.
Cisco
Documentation: [Link to Understanding and Monitoring Environmental Alerts
on Nexus Switches]