Dell R740 Network Failure. Broadcom NDC ports blink but won't connect in Proxmox

Hi everyone,

I am setting up a Dell EMC PowerEdge R740 with Proxmox VE 8. I have run into a specific networking wall that I can't seem to break, and I'm looking for a "final check" before I swap out hardware.

The Situation:

• Success: I can install and run Proxmox perfectly if I use a USB-to-RJ45 adapter. The system installs, boots, and gets internet access without issue via the USB adapter.

• Failure: The moment I try to use the onboard Broadcom BCM5720 Quad-Port NDC, I get nothing.

  • The ports are physically alive: The link lights (heartbeat) are blinking on the server backplane when cables are plugged in.

  • Proxmox is blind: The OS simply does not pass traffic or seemingly initialize these ports correctly, even though the hardware lights are active.

The Hardware:

• Server: Dell PowerEdge R740

• Problematic NDC: Broadcom BCM5720 Quad-Port 1GbE (DP/N: 0FM487)

• OS: Proxmox VE 8 (Debian 12 Bookworm)

What I have already tried (using the USB adapter to download packages): I assumed this was a missing driver issue, so I have already done the following:

1. Drivers/Firmware: Enabled non-free-firmware repos and installed firmware-misc-nonfree (for the tg3 driver), firmware-bnx2, and firmware-bnx2x.

2. Updates: Ran update-initramfs -u and rebooted multiple times.

3. Network Config: Verified ip link to see if interfaces were just renamed (e.g., eno1 vs ens2f0)—I updated /etc/network/interfaces to match whatever appeared, but they remain DOWN or NO-CARRIER.

4. Dell Lifecycle Controller: Updated the NDC firmware to the latest version provided by Dell.

5. BIOS Settings: Ensured "Integrated Devices" is Enabled, checked IOMMU settings, and verified the card is physically seated correctly.

My Conclusion & Question: Since the server works fine via USB, I know the motherboard/CPU/Proxmox install is healthy. The issue is strictly this Broadcom NDC refusing to play nice with Proxmox VE 8.

I am planning to simply buy an Intel I350-t4 or Intel X550 NDC to replace this Broadcom card.

1. Is this Broadcom BCM5720 known to be incompatible or "broken" on Proxmox 8?

2. Is there one final "magic" fix I missed (maybe a specific GRUB parameter or Blacklist)?

3. Has anyone successfully swapped this specific Broadcom card for an Intel one on an R740 and had it work plug-and-play?

Thanks for any advice!