ConnectX Card Showing ib0 Instead of Ethernet? How to Switch a VPI Card With mlxconfig

If a ConnectX VPI card such as the MCX556A-ECAT or MCX653105A-HDAT shows up as ib0, or won't link to an Ethernet switch, its ports are set to InfiniBand. NVIDIA's documentation notes that both ports on a ConnectX-5 VPI card default to InfiniBand. The fix takes two commands and a reboot: mlxconfig -d <device> set LINK_TYPE_P1=2 LINK_TYPE_P2=2, where 1 = InfiniBand and 2 = Ethernet. Ethernet-only (EN) cards don't have this setting and can't run InfiniBand.

Below: the step-by-step, how to tell VPI from EN, the cable that goes with each mode, and the mistakes that cost an afternoon.

Step by Step (Linux)

1. Install the tools. Either NVIDIA's MFT package (mst, mlxconfig) or the open-source mstflint package (mstconfig). On Ubuntu: sudo apt install mstflint.
2. Find the card. lspci | grep Mellanox. With MFT, run sudo mst start and sudo mst status to get a device path such as /dev/mst/mt4119_pciconf0 (mt4119 is ConnectX-5, mt4123 is ConnectX-6, mt4129 is ConnectX-7).
3. Check the current setting. sudo mlxconfig -d /dev/mst/mt4119_pciconf0 q | grep LINK_TYPE. With mstflint, use the PCI address: sudo mstconfig -d 0000:3b:00.0 q.
4. Set Ethernet. sudo mlxconfig -d <device> set LINK_TYPE_P1=2 LINK_TYPE_P2=2. Answer y at the prompt; the default answer is no.
5. Reboot. The new setting takes effect on the next boot.
6. Verify. Run ip link and check that an Ethernet interface appears in place of ib0, then run the query from step 3 again.

To go the other way, set the value to 1. Mixed mode works on dual-port VPI cards: LINK_TYPE_P1=1 LINK_TYPE_P2=2 gives one InfiniBand port and one Ethernet port.

VPI or EN? Read the Part Number

• VPI (either protocol): MCX555A-ECAT, MCX556A-ECAT, MCX653105A-ECAT, MCX653105A-HDAT, MCX653106A-HDAT, MCX755106AS-HEAT
• EN (Ethernet only): MCX515A-CCAT, MCX516A-CCAT, MCX516A-CDAT, MCX623106AN-CDAT, MCX713106AS-CEAT, plus the 25G SFP28 cards

The full pattern is in our ConnectX part-number decoder.

Four Mistakes That Cost an Afternoon

1. Not rebooting. The query shows the new value as pending, but nothing changes until the card restarts.
2. Using an InfiniBand-coded cable on an Ethernet switch. Many Ethernet switches reject cables that aren't coded for Ethernet. For 100GbE use an Ethernet-coded DAC such as the MCP1600-C001 (new, $45); keep EDR InfiniBand cables such as the MCP1600-E001E30 for InfiniBand links.
3. Changing only one port on a dual-port card and then troubleshooting the other one.
4. Treating it as a hardware fault. Dell's support article on ConnectX-5 links that go down after a card replacement explains that the replacement VPI card arrives in InfiniBand mode, and the fix is the same mlxconfig change and reboot. A replacement card doesn't inherit the old card's settings.

If the port type is right and the link still won't come up, move to the physical layer with our mlxlink and mst field guide and the FEC mismatch checklist.

FAQ

Why does my ConnectX-5 show up as ib0 instead of an Ethernet interface?
It's a VPI card, and its ports are set to InfiniBand. NVIDIA's documentation notes that both ConnectX-5 VPI ports default to InfiniBand. Set LINK_TYPE_P1 (and P2 on a dual-port card) to 2 with mlxconfig or mstconfig, then reboot.

What are the LINK_TYPE values?
1 = InfiniBand, 2 = Ethernet. The setting is per port, so a dual-port VPI card can run one port as InfiniBand and the other as Ethernet.

Do I need to reboot after changing LINK_TYPE?
Yes. The setting is stored in the card's firmware configuration and takes effect at the next reboot (or after an mlxfwreset where supported). Check with mlxconfig -d <device> q | grep LINK_TYPE afterwards.

Can I switch an Ethernet-only ConnectX card to InfiniBand?
No. EN cards such as the MCX515A-CCAT, MCX516A-CCAT, MCX623106AN-CDAT and MCX713106AS-CEAT are Ethernet only. For InfiniBand you need a VPI card such as the MCX556A-ECAT (refurbished, $244.95) or MCX653105A-HDAT (refurbished, $619.95).

Which ConnectX cards are VPI?
The ones with an E or H speed letter on a VPI family: MCX555A-ECAT, MCX556A-ECAT, MCX653105A-ECAT, MCX653105A-HDAT, MCX653106A-HDAT and MCX755106AS-HEAT. Our ConnectX part-number decoder lists all of them.

Do I need a different cable for Ethernet and InfiniBand?
Often, yes. Ethernet switches check the cable's coding, and EDR InfiniBand-coded cables are not always accepted on an Ethernet switch. For 100GbE use an Ethernet-coded DAC such as the MCP1600-C001 (new, $45); for EDR InfiniBand, the MCP1600-E001E30 (new, $42).

Ready to Buy?

VPI cards, plus the right cable for each mode. Lifetime warranty, shipping from Knoxville, Tennessee. Prices below checked October 22, 2026.

• MCX555A-ECAT — ConnectX-5 VPI, 1 × EDR / 100GbE — $149.95, refurbished, in stock at Resilient Tec; InfiniBand or Ethernet
• MCX556A-ECAT — ConnectX-5 VPI, 2 × EDR / 100GbE — $244.95, refurbished, in stock at Resilient Tec; ports set independently
• MCX653105A-HDAT — ConnectX-6 VPI, 1 × HDR / 200GbE — $619.95, refurbished, in stock at Resilient Tec; HDR InfiniBand or 200GbE
• MCX755106AS-HEAT — ConnectX-7 VPI, 2 × 200G QSFP112 — $1,499, refurbished, in stock at Resilient Tec; NDR200 or 200GbE
• MCP1600-C001 — 100GbE QSFP28 passive DAC, 1 m — $45, new, in stock at Resilient Tec; Ethernet mode
• MCP1600-E001E30 — EDR InfiniBand QSFP28 passive DAC, 1 m — $42, new, in stock at Resilient Tec; InfiniBand mode

You Might Also Need

• How to read a ConnectX part number — every MCX suffix decoded
• mlxlink and mst field guide — reading a ConnectX link that won't come up
• DAC cable no link? — fixing 25G/100G FEC mismatches
• InfiniBand vs Ethernet for AI clusters — which fabric fits your stack
• 100G QSFP28 DACs for AI homelab clusters — the ConnectX cable guide

Sources

• NVIDIA — Port type management (LINK_TYPE_P1/P2)
• Dell — Mellanox MT27800 (ConnectX-5) link is down after replacement