The following how-to will describe the different headless boot modes that are achievable on an Intel Xserve using a combination of the power button and the system identifier button the exclamation point inside the triangle.
via Apple Intel Xserve Boot Modes « bn technical support forum.
- In order to enter the headless command mode, hold the identifier button while you power the Xserve on.
- Keep pressing the identifier until you see the top row of blue lights on the Xserve blinking.
- Stop pressing the identifier button. You’ll notice that the right-most blue LED on the bottom row is lit up - this indicates that the system will start from the system disk.
- At this point you can press the system identifier button in order to cycle through the boot options. They are as follows right to left- System Disk- Network Boot- Start from leftmost drive- Boot next disk other than startup disk- Target Boot Mode- Reset PMU
- [press and hold for several seconds and] Release the button once again and the system will boot into your desired mode.