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[BTI] BTI7800 FPSD interoperation issue with third vendor device

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Article ID: KB35649 KB Last Updated: 04 Apr 2020Version: 2.0
Summary:

When a fault is raised against a pluggable transceiver interface, the corresponding transmitting laser on the transceiver at the far end of the link continues to function and can transmit unreliable information.

Fault propagation shutdown (FPSD) provides a means to quickly shut down a transmitting laser and pass the fault to the downstream device when a receiver signal failure occurs. When FPSD is enabled, the transmit laser is shut down in a fault scenario. 

Diagram below explains how FPSD is triggered when link fault is detected at one end. With FPSD enabled at both ends ,when the left end 7800 client interface detect link fault, it will send “Local Fault” alarm to the downstream node (the right end). This will trigger FPSD to laser off the interface and the peering device will send “Remote Fault” back to the 7800 devices. Once the link fault clears, the downstream 7800 interfaces will laser on again to restore the service.



 
Symptoms:

When the BTI7800 interoperatse with some third party vendor's device, FPSD function may cause both ends of the 7800 client interfaces to turn the laser off and the link will remain down even after the link fault clears. To restore the link, the customer has to temporarily disable FPSD on the client interface at both ends.

This article explains how this happens and solution to this issue.

Cause:
For BIT 7800 client ethernet interfaces, the link fault which triggers FPSD engaged includes three alarms:
  •  LOS (Loss of Signal), 
  •  LOSync(Loss of Sync)
  •  LF(Local Fault). 

By default, when 7800 client interface laser off triggered by FPSD, it is expected to receive "Remote Fault" from the peering device, as RF doesn't not trigger FPSD which is depicted in diagram above. 

Investigation shows that, when some third vendor's device in "Loss of Signal" state, it sends out "Loss_of_Sync" to 7800 instead of "Remote Fault" alarm. As  LOSync alarm will cause will trigger FPSD engaged and cause far end BTI7800 to laser off its client interface. this will cause both end BTI7800 interface FPSD engaged and laser off the interfaces at both ends.

When link fault detected at one end, FPSD at both ends will fall into inter-locked state and laser off the interfaces at both ends.  Under such condition,  Link will not be restore again even the original fault has been cleared. Customer must manually disable FPSD at both ends to restore the link. 

The diagram below shows how FPSD inter-lock state happens:

 
Solution:

BTI7800 FPSD function works normally with BTI7000 series devices. But FPSD inter-lock state issue happens when BTI7800 interoperates with BTI821 devices and some other third vendor's dev.

Prior to enabling FPSD function on BTI7800 device, it is recommended to verify what alarm the peering device interface will send to BTI7800 back when LOS alarm is raised at the this device. If it is a LoSync alarm sent back to BTI7800, then FPSD shall not be enabled in such condition to avoid the inter-lock state. 

When FPSD disables client interface, if link fault is detected at one end, the far end 7800 send out "Local Fault" to the connected device instead of laser off the interface.

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