Why does Spectrum show Cisco Stack Ports when they do not exist on the Cisco Catalyst Switch?

Products

• Network Observability
• Spectrum

Issue/Introduction

We have noticed a few alarms of "Bad Links Detected" that were being created by Spectrum on the Cisco Stacksub interfaces, however these interfaces do not exist in the switch and we are not sure why Spectrum is alarming on them?

Environment

Spectrum ANY

Cisco Catalyst Devices

Cause

• Internal Architecture of Cisco Stack Ports:
• Cisco Catalyst switches use hardware-based stacking (even on standalone devices).
• IOS pre-creates logical interfaces for stack communication: StackPortX (physical) and StackSub-St1-1/StackSub-St1-2 (virtual).
• These are registered in the IF-MIB for SNMP monitoring.
• Why They Stay Admin-Up:
• Cisco designs them to remain administratively "up" for immediate stack readiness.
• There is no CLI or SNMP command to shut down these virtual interfaces.
• Oper-Down Status:
• When no stack cable is connected, OperStatus is "down" (physical link inactive).
• AdminStatus remains "up" (system expects readiness).
• This "admin-up / oper-down" mismatch triggers alerts in monitoring systems.
• Why They Exist in SNMP Even Without Stack:
• Cisco IOS populates the IF-MIB with all potential interfaces at boot.
• Removing them dynamically would break SNMP indexing consistency.
• Why You Can't Disable Them:
• These virtual interfaces are not exposed for configuration (commands only affect the physical StackPort).
• They are software constructs tied to the stacking feature.
• Impact on Monitoring:
• Monitoring tools (Zabbix, Spectrum, SolarWinds) interpret "admin-up / oper-down" as a fault.
• They cause persistent false positives unless explicitly filtered or excluded from monitoring.
• Summary: These "phantom" interfaces are inherent to Cisco's stacking architecture, are SNMP compliant, remain admin-up (but oper-down when unused), and cannot be disabled. Monitoring requires excluding them or adjusting alert logic to avoid false positives.

Resolution

Although these interfaces do not exist on the device, they do exist in the mib.

If you have no need and no plan to use these stackports then you can enable on the port interface model GeneratePortStatusAlarms to No as mentioned in this kb article.

However if you think that it is possible that someone will plug a cable into this port in the future and use this interface, then you can place the interface into hibernation mode as mentioned here.