Troubleshoot FXOS Fault F1758 External-Port-Link Conflicts on Firepower 4115 FTD

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Updated:March 30, 2026

Document ID:225672

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Issue

A Firepower 4115 FTD device in an HA configuration is displaying FXOS major fault F1758 on external-port-links PC2_ftd and PC7_ftd. The fault messages indicate conflicts with the application referencing unknown.

The issue is particularly confusing because the referenced sub-interfaces are not visible in the FMC UI for the FTD interfaces page, and no traffic impact has been observed. The fault became visible after enabling interface health monitoring on the FMC.

Environment

Firepower 4115 FTD in HA configuration
FTD Software Version: 7.4
FXOS Software Version: 2.14.2.137
Interface health monitoring recently enabled on FMC
External-port-links PC2_ftd and PC7_ftd configured
No traffic impact or service interruption observed

Resolution

The FXOS fault F1758 on external-port-links PC2_ftd and PC7_ftd is a known cosmetic software defect (CSCvr17111) that does not impact traffic forwarding or device operation. This fault affects only the fault reporting mechanism and can be safely ignored if no functional impact is observed.

Verification Steps

1: Confirm the fault matches the known defect pattern by verifying the fault details match CSCvr17111 behavior where unknown VLAN sub-interfaces referenced in the fault are not actually configured on the FTD.

ld/firepowerdevice/ext-portlink/PC2_ftd/fault/F1758
Description:
The external-port-link PC2_ftd conflict with the application. Port-channel2: interface not present for update! (vlan = 1002, innerVlan = -1). To correct it, synchronize and remove the conflict sub-interface, save and deploy from Firepower Management Center. Remove and recreate external-port-link PC2_ftd from MIO, then, synchronize it again from Firepower Management Center.
--
ld/firepowerdevice/ext-portlink/PC7_ftd/fault/F1758
Description:
The external-port-link PC7_ftd conflict with the application. Port-channel7: interface not present for update! (vlan = 1007, innerVlan = -1). To correct it, synchronize and remove the conflict sub-interface, save and deploy from Firepower Management Center. Remove and recreate external-port-link PC7_ftd from MIO, then, synchronize it again from Firepower Management Center.

2: Verify no traffic impact or unexpected failover behavior is occurring in the HA environment.

3: Confirm the timing of fault appearance correlates with enabling interface health monitoring rather than a functional configuration change.

Resolution Options

1: Accept Cosmetic Fault (Recommended)

Since the F1758 fault is cosmetic and causes no functional impact, it can be safely ignored. Monitor the system for any actual traffic issues or unexpected behavior, but no immediate action is required.

2: Clear the Fault (If Operationally Necessary)

If the cosmetic fault creates operational confusion or alarm noise, follow the official FXOS F1758 remediation steps:

Synchronize and remove any conflict sub-interface from Firepower Management Center (if present).
Save and deploy the changes from Firepower Management Center.
Remove and recreate the affected external-port-links in FXOS (MIO).
Synchronize the device again from Firepower Management Center.

3: Reboot Method

Plan a maintenance window and reboot the affected device, as the fault must clear automatically after a system restart.

Prevention and Monitoring

Continue using FMC interface health monitoring to watch for genuine link or port-channel issues. Open a new TAC case if traffic impact, unexpected failovers, or new fault codes appear in conjunction with F1758. Keep FXOS/FTD software at current recommended releases where cosmetic defects must be reduced or resolved.

Cause

The FXOS fault F1758 is caused by Cisco Bug ID CSCvr17111, a known cosmetic software defect in FXOS/FTD. This defect causes the system to incorrectly report external-port-link conflicts for VLAN IDs even when no such sub-interfaces are configured or in conflict on the FTD. The condition affects only fault reporting mechanisms and does not impact data-plane forwarding or device operation.