Packet loss and CRC errors and changes of interface state

Two routers are connected back-to-back, and a ping between them is successful, but sometimes there are large amount of packets lost and frequent CRC errors, or the interface state alternates between UP and DOWN.

Check the ATM interfaces of the two nodes to see if their types are the same (both are multimode fiber interface or both are single-mode fiber interface). If their types are different, you should change one of them. In most cases, when a multimode fiber interface and a single-mode fiber interface are directly connected, they can communicate, but sometimes with the mentioned faults.