Are you sure that the SFP is a Multimode? It sounds like a singelmode? Cisco GLC-BX-U20 Compatible 1000BASE-BX-U SFP BiDi Transceiver Module (SMF, 1310nm-TX/1490nm-RX, 20km, LC, DOM) And also if you have the BX-U in one end, you need the BX-D in the other end.
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