CCNA Voice Tutorial – DHCP and the IP Phone Boot Process

by Chris Bryant - 15 October 2009

Just as our switches and routers have certain tests and procedures they run when they boot, so do Cisco IP Phones – but of course, there’s a major difference or two between the Phone and router boot processes!

Here’s the basic boot procedure for a Cisco IP Phone:

As you’d expect, the first step is to physically connect the Phone to a switch port. The Phone can receive its power via Power Over Ethernet (PoE) if the Phone and Switch agree on the PoE method to be used. Originally called “inline power”, PoE allows the power to be sent through the copper cable to the connected device.

Our old friend Cisco Discovery Protocol enters the picture now, as its CDP that tells the Phone what Voice VLAN to use. Be sure CDP is running on the switch port that’s connected to the IP Phone, as it’s commonplace to disable CDP.

The Phone now needs an IP address – after all, an IP Phone will not be much good without one! That address and other related information is acquired in much the same way a “regular” host device does so – via DHCP. The IP Phone sends a DHCP Request, which is forwarded by the router to the DHCP Server.

That forwarding required the router to be configured with the ip helper-address command; routers do not forward broadcasts by default.

From there, the process is much like a PC receiving an IP address – the DHCP server responds with the usual Offer, and the Phone accepts the first Offer it receives.

This process isn’t exactly like a PC requesting an address via DHCP, though. With IP Phones, there’s a little extra option in DHCP – Option 150, to be exact. This Option tells the IP Phone where to find the TFTP Server that holds the Phone’s configuration file and is a vital part of IP Phone operation.

The Phone then contacts that TFTP server and then downloads its configuration file. From the contents of that config file, the Phone learns the location of the call processing agents it should use, and then attempts to contact them in the order in which they’re listed in the file.

If you’re configuring a Cisco router as a DHCP Server, be sure to enable Option 150 – and I’ll show you just how to do that in the next CCNA Voice tutorial in this series.

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNA certification exam tutorials.
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