The interface remain in physical status down.

MDU MA5671A (AIR ONT)  connects to a S5700-28P-LI-AC through SFP ONT. At GPON side, the MDU is ONLINE, it is in the Switch were the Gigabit Ethernet port is DOWN. Check below the interface status:

<lab-switch6>display interface GigabitEthernet 0/0/28
GigabitEthernet0/0/28 current state : DOWN
Line protocol current state : DOWN
Description:
Switch Port, Link-type : trunk(configured),
PVID :   10, TPID : 8100(Hex), The Maximum Frame Length is 9216
IP Sending Frames' Format is PKTFMT_ETHNT_2, Hardware address is 688f-8403-382c
Current system time: 2016-04-28 15:49:26
Port Mode: COMMON FIBER, Transceiver: 1000_BASE_LX_SFP
Speed : 1000,   Loopback: NONE
Duplex: FULL,   Negotiation: ENABLE
Mdi   : NORMAL, Flow-control: DISABLE
Last 300 seconds input rate 0 bits/sec, 0 packets/sec
Last 300 seconds output rate 0 bits/sec, 0 packets/sec
Input peak rate 0 bits/sec, Record time: -
Output peak rate 0 bits/sec, Record time: -

Input:  0 packets, 0 bytes
  Unicast:                          0,  Multicast:                           0
  Broadcast:                        0,  Jumbo:                               0
  Discard:                          0,  Pause:                               0
  Frames:                           0

  Total Error:                      0
  CRC:                              0,  Giants:                              0
  Jabbers:                          0,  Fragments:                           0
  Runts:                            0,  DropEvents:                          0
  Alignments:                       0,  Symbols:                             0
  Ignoreds:                         0

Output:  0 packets, 0 bytes
  Unicast:                          0,  Multicast:                           0
  Broadcast:                        0,  Jumbo:                               0
  Discard:                          0,  Pause:                               0

  Total Error:                      0
  Collisions:                       0,  ExcessiveCollisions:                 0
  Late Collisions:                  0,  Deferreds:                           0
  Buffers Purged:                   0

    Input bandwidth utilization threshold : 90.00%
    Output bandwidth utilization threshold: 90.00%
    Input bandwidth utilization  :    0%
    Output bandwidth utilization :    0%

The first thing to do is to check if transceiver is properly plugged and if it receives the required amount of signal. Check below command, we can see that the signal value for TX and RX are inside the signal thresholds. So we will need to

[lab-switch6] display transceiver interface GigabitEthernet 0/0/28 verbose

GigabitEthernet0/0/28 transceiver information:
-------------------------------------------------------------
Common information:
  Transceiver Type               :1000_BASE_LX_SFP
  Connector Type                 :SC
  Wavelength(nm)                 :1310
  Transfer Distance(m)           :20000(9um)
  Digital Diagnostic Monitoring  :YES
  Vendor Name                    :HUAWEI         
  Vendor Part Number             :MA5671A        
  Ordering Name                  :
-------------------------------------------------------------
Manufacture information:
  Manu. Serial Number            :030XXD6TF1400236
  Manufacturing Date             :2015-08-13
  Vendor Name                    :HUAWEI         
-------------------------------------------------------------
Diagnostic information:
  Temperature(��C)              :32.29
  Temp High Threshold(��C)      :80.00
  Temp Low  Threshold(��C)      :-50.00
  Voltage(V)                    :3.34
  Volt High Threshold(V)        :3.60
  Volt Low  Threshold(V)        :3.00
  Bias Current(mA)              :5.76
  Bias High Threshold(mA)       :100.00
  Bias Low  Threshold(mA)       :2.00
  RX Power(dBM)                 :-13.37
  RX Power High Threshold(dBM)  :-6.00
  RX Power Low  Threshold(dBM)  :-28.86
  TX Power(dBM)                 :2.48
  TX Power High Threshold(dBM)  :5.99
  TX Power Low  Threshold(dBM)  :-0.50
  Transceiver phony alarm       : Yes
-------------------------------------------------------------

Since the signal level is correct, I suggest to the customer the following troubleshooting steps:

1. disable auto-negotiation, and set speed and duplex manually. 

[R5_U11_S5710-EI]int gi0/0/28
[R5_U11_S5710-EI-GigabitEthernet0/0/28]undo negotiation auto                    
[R5_U11_S5710-EI-GigabitEthernet0/0/28]speed 1000                               
[R5_U11_S5710-EI-GigabitEthernet0/0/28]duplex full           

2. Try to perform a software loopback on the physcal port. Check the status:

<HUAWEI> system-view
[HUAWEI] interface gigabitethernet 0/0/28
[HUAWEI-GigabitEthernet0/0/28] loopbacktest internal

3. Perform a physical loopback on the S5700 switch, and connect the same fiber segment on two ports on the same S5700 switch, in this way we can confirm if the port, transceivers and fiber is not faulty.

Following the troubleshooting steps we manage to find out that negotiation status on both ends is not consistent.

We disable negotiation on the S5700 interface and the link came up.

[lab-switch6]display interface GigabitEthernet 0/0/28
GigabitEthernet0/0/28 current state : UP
Line protocol current state : UP
Description:
Switch Port, Link-type : access(configured),
PVID :    1, TPID : 8100(Hex), The Maximum Frame Length is 9216
IP Sending Frames' Format is PKTFMT_ETHNT_2, Hardware address is 688f-8403-382c
Current system time: 2016-05-03 22:24:35+01:00
Port Mode: COMMON FIBER, Transceiver: 1000_BASE_LX_SFP
Speed : 1000,   Loopback: NONE
Duplex: FULL,   Negotiation: DISABLE
Mdi   : NORMAL, Flow-control: DISABLE
Last 300 seconds input rate 0 bits/sec, 0 packets/sec
Last 300 seconds output rate 2608 bits/sec, 2 packets/sec
Input peak rate 232512 bits/sec, Record time: 2016-05-03 20:52:26
Output peak rate 41840 bits/sec, Record time: 2016-05-03 20:43:24

Input:  1936 packets, 257920 bytes
  Unicast:                        184,  Multicast:                        1660
  Broadcast:                       92,  Jumbo:                               0
  Discard:                          0,  Pause:                               0
  Frames:                           0

  Total Error:                      0
  CRC:                              0,  Giants:                              0
  Jabbers:                          0,  Fragments:                           0
  Runts:                            0,  DropEvents:                          0
  Alignments:                       0,  Symbols:                             0
  Ignoreds:                         0

Output:  53579 packets, 6255787 bytes
  Unicast:                        145,  Multicast:                       41905
  Broadcast:                    11529,  Jumbo:                               0
  Discard:                          0,  Pause:                               0

  Total Error:                      0
  Collisions:                       0,  ExcessiveCollisions:                 0
  Late Collisions:                  0,  Deferreds:                           0
  Buffers Purged:                   0

    Input bandwidth utilization threshold : 90.00%
    Output bandwidth utilization threshold: 90.00%
    Input bandwidth utilization  :    0%
    Output bandwidth utilization :    0%

Try to get information about interface status from both equipments, otherwise you will have to perform different operations one by one, which may become very time consuming.