The possible cause is that the types of the optical fibers used on the ATM interfaces at both ends are different. The multi-mode optical fiber is used at one end while the single-mode optical fiber is used at the other end.
NOTE: In most cases, the multi-mode optical interface and the single-mode optical interface that are directly connected can communicate with each other. Sometimes, packet loss and CRC errors may occur or the interface converts between Up and Down continuously.
Check whether the ATM interfaces at both ends are of the same type, that is, multimode optical fiber interface or single-mode optical fiber interface.
