The Remote Power Button
PXE handles the software side of provisioning, but there is a prerequisite: the server must actually be powered on and configured to boot from the network. How do you power on 500 servers in a remote data center? How do you access the BIOS to change the boot order? How do you reboot a server whose OS has completely crashed and is not responding to SSH?
The answer is Out-of-Band (OOB) Management — a separate management channel that is completely independent of the main server's operating system.
The Baseboard Management Controller (BMC)
Inside every enterprise server is a small, dedicated microcontroller called the Baseboard Management Controller (BMC). Think of it as a tiny computer-within-a-computer. It has its own CPU, its own RAM, its own firmware, and critically, its own dedicated physical network port on the back of the server.
The BMC runs 24 hours a day, 7 days a week, as long as the server has power — even when the main server is "off." It is completely independent of the main CPU, RAM, and operating system.
Through the BMC's dedicated network connection, you can remotely:
- Power cycle the server (hard reset, graceful shutdown, power on from cold).
- View a virtual console (KVM) — see exactly what is on the server's monitor, including BIOS screens, before any OS loads.
- Mount virtual media — attach an ISO file from your laptop as if it were a physical CD-ROM plugged into the server.
- Read hardware sensors — CPU temperature, fan speeds, power consumption.
- Update firmware — flash new BIOS or BMC firmware without touching the OS.
The Protocols: IPMI vs. Redfish
IPMI (Intelligent Platform Management Interface) is the older standard, defined in the 1990s. It uses a binary protocol over UDP port 623, driven from the command line by the standard ipmitool utility — power cycling, changing next-boot device, and reading the hardware event log are all single invocations authenticated against the BMC's IP and credentials.
Redfish is the modern replacement, standardized by the DMTF consortium. It exposes the same BMC capabilities as a RESTful HTTP API with JSON payloads — the same power-on or set-next-boot-to-PXE operations become ordinary HTTP requests instead of a specialized binary protocol, which makes them dramatically easier to automate with standard tooling and libraries most engineers already know.
A common interview question is: "What is the difference between in-band and out-of-band management?" The answer: In-band management uses the server's primary network interface and requires the OS to be running (SSH, for example). Out-of-band management uses a separate physical network connection to the BMC and works regardless of OS state — even when the server is powered off or completely crashed. The practical implication: if you can only SSH into a server, you have no recourse when the OS hangs. If you have OOB access, you can always recover.
Key Point: IPMI and Redfish are not interchangeable in practice. IPMI is a binary protocol with known security vulnerabilities (cipher suite weaknesses, replay attacks) and should be isolated on a dedicated management VLAN with no internet exposure. Redfish is the direction the industry is moving — it is easier to script, supports modern authentication, and is vendor-neutral. When building a new provisioning system, prefer Redfish where hardware supports it.