Topology: NE80 and S8505 are connected with GE port single-mode.
Phenomenon: The physical and protoocl layer of GE port in NE80 side is down suddenly. The physical layer of GE port in S8505 side is up, but the link layer is down.
Have remote loopback test at NE80 and S8505. The GE interface of two devices is up. The physical layer of S8505 is up, so link interface and physicl interface can be ironed out. And then check statistics information of connected interface of NE80 and S8505 and it is found that input packet has much CRC. The negotiation is problematic. Check it and GE of both sides is self-negotiation. Cancel self-negotiation mode of both sides and make it as forced Gigabit full-duplex. And then check interface of NE80 and S8505. It is up. Test link recovers and the problem is solved.
1. Tail fiber link is problematic.
2. The optical module of the interface is problematic.
3. The port negotiation is problematic.
Generally GE is forced Gigabit full-duplex. For the conenction of self-negotiation mode, change the mode as forced mode to avoid the problem.