Diagnosing a ConnectX-4 port with link but no traffic and 25G optics negotiating only 1G on a MikroTik switch
With two dual-port Mellanox ConnectX-4 cards, a MikroTik SFP+ switch, 1 m OM4 patches and mixed 10G/25G transceivers from several vendors, one NIC port showed link UP but got no DHCP and could not route, 25G transceivers negotiated only 1G, and previously stable 10G links dropped intermittently. Swapping transceivers and fibres, forcing speed with ethtool and rebooting gave inconsistent results.
- Hardware: Mellanox ConnectX-4, MikroTik SFP+ switch
- Transceivers: 10G SFP+ SR, 25G SFP28 SR (mixed vendors), SFP+ DAC
- Form factor: SFP+, SFP28, DAC
Symptoms / messages:
link UP but no DHCP
25G optic negotiates 1G
intermittent link loss
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Comments 2
Isolate by swapping transceivers and fibres between ports (a single damaged strand can cause it), use a DAC as a simple reference link, update NIC firmware, inspect the card edge connector, and confirm on the DHCP server whether requests arrive.
Testing with network namespaces finally produced 23.5 Gbps with the 25G optics and the second port proved fine; root cause stayed unclear but looked transceiver/environment related.