For the slightly more initiated, the rough process is as follows:
Overview of IP Phone Boot Process
1. Pre-802.3af devices use FLP (Fast Link Pulse) to detect if power should be applied.
2. 802.3af devices use a DC-voltage detection mechanism to determine if power should be applied.
3. Optionally, 802.3af devices can perform powered device classification to determine how much power the device really needs.
4. Once the device is powered, the phone uses either Cisco Discovery Protocol (CDP) or Link Layer Discovery Portocol-Media Endpoint Discovery (LLDP-MED) to determine the VLAN ID to use. Please note that LLDP-MED is supported in IP Phone firmware versions 8.3(3) and later.
5. Using the correct VLAN ID, the phone then attempts to get IP configuration information from a DHCP server (unless IP information is statically configured). The DHCP response Option 150 should have the TFTP server information included.
6. The phone then attempts to access the TFTP server supplied by the DHCP server to get the phone's configuration information, image loads, etc.
7. Assuming all is good in the world, registration ensues.
IEEE 802.3af-2003 describes five (5) power classes that a device may belong to. It should be noted that it is not mandatory that a PSE vendor implement power classification, as these classifications are optional. An important consideration when deploying a Power over Ethernet solution is how power is managed, and the resources, for example power draw, cooling, etc, that the solution requires to operate. If a vendor chooses not to implement power classification, the default power classification within IEEE 802.3af delivers 15.4W per power device that may require a network or facilities manager to invest in more power and cooling resources than a Cisco Power over Ethernet solution that implements intelligent power management. It should also be remembered that even though a powered device may support IEEE 802.3af-2003 power classification, the PSE may not, and 15.4Watt delivery is the lowest common denominator. Additionally, even if the PSE and powered device support power classification, the classification ranges are fairly broad that can lead to wasted power budget allocation. Refer to Table 1 for details of IEEE 802.3af-2003 power classes.
Folks, this post is really just a very brief summary of the basics involved in supplying power to an IP phone via PoE. This Cisco document is EXCELLENT - and I highly recommend that it be reviewed closely to clear up a lot of misconceptions related to PoE.
Also, this document is an extensive discussion of the differences between CDP and LLDP-MED. Aspiring network and voice engineers, if you've never read this document, I guarantee that you'll be surprised with how much information is exchanged by these discovery protocols - read it!