Description of the problem

A customer's transmission network has two OSN3500 devices, A and B, with three STM-1 optical paths connected to each other, and uses EGS4 single boards to open Ethernet services between these two points using one VCTRUNK each. Considering service security, the VC12 channels bound to the VCTRUNKs of A and B are configured on the three STM-1 optical paths respectively, and the LCAS function is enabled.

On a certain day, the first STM-1 optical path between A and B has an abnormality with a large number of BERs, causing the two devices to frequently report LCAS_PLCT/LCAS_PLCR alarms. In order to locate the problem, the customer made an inner loopback (loop to A) to the first STM-1 optical path, and the EGS4 veneer started to report LCAS_TLCT/LCAS_TLCR alarms, and the Ethernet service was interrupted.

Processing

After canceling the loopback, operations resume.

Root cause

The LCAS protocol is built on top of VCTRUNKs, and VCTRUNKs between two network elements must be one-to-one, not one-to-many. If a customer performs an inner loopback on the part of the channel bound by VCTRUNK in network element A, it is equivalent to the VCTRUNK of network element A corresponding to two VCTRUNKs: one consists of the looped back channel and the other consists of the un-looped back channel. The LCAS protocol then randomly selects one of them and considers the other one unavailable. If the selected channel is the unlooped channel, then LCAS_PLCT and LCAS_PLCR alarms will appear. If the selected channel is the loopback channel, since there will be an instantaneous break in the loopback operation, LCAS_TLCT and LCAS_TLCR will be reported, and the WTR will be entered immediately, but the whole service is actually broken.

Solution

Business resumed after the loopback was canceled.

Recommendations and summary

The problem is the same for all data veneers. Therefore, it is recommended to disable the loopback operation for the portion of the notification bound to a particular VCTRUNK after starting the LCAS.

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