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A few of my recent systems have come with a built-in combo BMC on the motherboard NICs. I haven't seen this before, is it a new trend? I'm imagining they put in a switch in there with the NICs? How does this even work?

I'm doing a few timing sensitive projects involving hardware timestamping in the NICs. Does this mess with say timing variability? I've disabled the BMCs out of paranoia but I don't know the topology inside.

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NC-SI. It allows the BMC and the host to share a single physical network connection.

That's common, and something to avoid. You want a physically separate BMC network for security and reliability.

One example: on some servers turning off NIC will... turn off the port completely, including shared BMC connection hanging off it.

Which happens during reboot meaning you won't catch the boot screen on KVM if you use shared nic

What I remember was losing it just at a crucial point in the PXE boot process. You're also dependent on the same switches as the main net, and have to rely on fragile firewalling or VLANs to restrict access to the BMCs.

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