I had a customer which have two 3550 in building A and two 3550 in building G. From A to G they are connected with 1000 Base LH/LH. From all those 3550 they have about sixteen 2950 connected with 1000BaseT or 1000Base SX. The hole are spilt in two cluster(Cluster A and Cluster G). After upgrading those clusters from 12.1(9)EA1c to 121-13.EA1a the connecting between A and G with 1000 Base LH/LH become flapping interface between 3550 many times each day. During the troubleshooting i notice that the linklight became orange -green-orange-green and so on. The customer have one multimode fiber so I rent them 2 1000Base SX, so the problem are temporary solved. What shall I do to go back to they own LX/LH with singel mode fiber.