Configuration Guide for Cisco NCS 1001, IOS XR Release 25.x.x

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Configuration Guide for Cisco NCS 1001, IOS XR Release 25.x.x

PSM revertive switch

Updated: July 27, 2026

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This topic describes how PSM revertive switching returns traffic to the primary path after a fiber cut clears and explains WTR and threshold hysteresis behavior.

When a loss of signal (LOS) alarm occurs on the primary path because of a fiber cut, traffic moves from the primary path to the secondary path. The PSM revertive switch feature allows traffic to return to the primary path when the fiber cut is resolved and the LOS alarm is cleared. This switch back to the primary path is not immediate; it depends on several parameters.

Parameters of PSM revertive switch

  • WTR (Wait To Restore): WTR is the time delay, in seconds, introduced after the LOS alarm on the primary path is cleared. When the WTR timer elapses, traffic moves to the primary path.

  • Threshold hysteresis: The threshold hysteresis parameter prevents transient or fluctuating power readings near the threshold monitoring the primary RX port.

    If set, the threshold hysteresis parameter works with the WTR timer. After the fiber cut is fixed and the alarm is cleared, the system starts the WTR timer only when the power on the primary RX port is higher than the combined value of the rx-low-threshold and threshold hysteresis (rx-low-threshold-delta).

Note

The suggested values for WTR and threshold hysteresis parameters are 120 seconds and one dBm, respectively. You must choose both parameter values appropriately based on conditions, such as the power received, the type of PSM protection scheme, whether manual threshold values are provisioned on PSM, or if auto-threshold is enabled.

Limitations

  • PSM revertive switch feature is supported only on section protection or path protection schemes.

  • PSM revertive switch feature is not supported on PSM three-way protection scheme.

  • PSM revertive switch feature is not supported on section protection or path protection schemes involving one or more ILA nodes.

  • The PSM revertive switch feature is triggered only when traffic switches from the primary path to the secondary path because of a fiber cut. It does not activate if the switch occurs due to user commands, such as manual-to or lock-out-from.

Configure a PSM revertive switch

Procedure

1.

Use this command to configure PSM revertive switch.

hw-module location 0/RP0/CPU0 slot psm-slot-number psm revertive wtr wtr-value primary-path path

wtr-value must be an integer. The primary path is set to WORKING by default. The user can change this path from WORKING to PROTECTED but cannot delete it.

Example:


configure
hw-module location 0/RP0/CPU0 slot 2 psm
revertive wtr 120
primary-path WORKING
commit
end
2.

Use this command to configure the threshold hysteresis for the selected OTS controller.

controller rack/slot/instance/port rx-low-threshold-delta rx-low-threshold-delta-value

rx-low-threshold-delta is the threshold hysteresis. The value 10 is expressed in units of 0.1 dBm. In this example, the value of threshold hysteresis is set as 1.0 dBm.

Example:


configure terminal
controller 0/2/0/1 rx-low-threshold-delta 10
commit
end
3.

Use this command to verify the revertive switch and threshold hysteresis values.

show controllers ots rack/slot/instance/port

Example:

RP/0/RP0/CPU0:ios# show controllers ots 0/2/0/1
        Controller State: Up 

       Transport Admin State: In Service 

       Port Type: Working 

       Port Status: Standby 

       Laser State: Unknown 
       
       Optics Status:: 

               Alarm Status:
               -------------
               Detected Alarms: 
                       
               Alarm Statistics:
               -----------------
               LOW-RX-PWR = 0          
               LOW-TX-PWR = 0          
               RX-LOS-P = 735        
               RX-LOC = 0          
               AMPLI-GAIN-DEG-LOW = 0          
               AMPLI-GAIN-DEG-HIGH = 0          
               AUTO-LASER-SHUT = 0          
               AUTO-POW-RED = 0          
               AUTO-AMPLI-CTRL-DISABLED = 0          
               AUTO-AMPLI-CFG-MISMATCH = 0          
               SWITCH-TO-PROTECT = 4          
               AUTO-AMPLI-CTRL-RUNNING = 0          
                
               Parameter Statistics:
               ---------------------
               TX Power = -16.70 dBm 
               RX Power = -21.00 dBm 
               RX Voa Attenuation = 0.00 dB 
               TX Voa Attenuation = 0.00 dB 
               TX Enable = Enabled 
               RX Enable = Enabled 
               Rx Low Threshold Current = -38.0 dBm 
               Wait Time to Restore = 120 secs 
                
                
               Configured Parameters:
               -------------
               Rx Low Threshold = -38.0 dBm 
               RX Voa Attenuation = 0.0 dB 
               TX Voa Attenuation = 0.0 dB 
               TX Enable = Enabled 
               RX Enable = Enabled 
               Rx Low Threshold Delta = 1.0 dBm

The configuration is committed and the optical module uses the requested settings.