Michael J Cavanaugh's Blog

How a Cisco IP Phone registers

mjcavanaugh — Sat, 01 Aug 2009 14:38:41 +0000

Have you ever wondered how a Cisco IP phone knows how to get its configuration and will keep that configuration even if you move it from one site to another? This is how:

When a Cisco IP Phone is plugged into a Cisco PoE switch it negotiates power consumption with the switch. (I will cover how this happens in more detail at a later date). Once power has been supplied, the phone and switch will exchange information through Cisco Discovery Protocol (CDP). Using CDP the phone will learn the tagged VLAN number (used for voice) and the untagged VLAN number (used for data). Once the phone knows the Voice VLAN number it will send out a DHCP discovery packet. A DHCP server will answer, instructing the phones to use a valid IP address on the voice VLAN. The DHCP server will also give the phone the IP address of a TFTP server, which is the Communications Manager server (CallManager). The phone will then send a packet to the CallManager and ask for its configuration. The CallManager will use TFTP to deliver the configuration to the phone. The phone reboots and loads the configuration.

As long as the phone can reach the CallManager server it can be moved anywhere in your IP network and still load its configuration and operate. Make sense?

Michael

Cisco’s Netpro Forum

mjcavanaugh — Mon, 06 Jul 2009 00:24:52 +0000

Have you ever been in the middle of an install of Cisco gear and ran into a snag? Someone else must have also hit this snag before. Wouldn’t it be nice if you could ask them what they did? With Cisco’s Netpro Forum you can:

http://www.cisco.com/go/netpro

Use the Netpro Forum to search past posts on various topics, such as Unified Communications, Security, Routing and Switching, etc. If you can not find a post that answers your questions, post the question yourself. More than likely, someone will answer you. If you see a post you know the answer to, please post in return.

Michael

My Intentions for this Blog

mjcavanaugh — Mon, 06 Jul 2009 00:17:10 +0000

Hello all,

Over the years I have picked up quite a few tip & tricks in designing and deploying Cisco solutions. In this blog I will attempt to explain and document them. Feel free to leave comments and share your thougts as well.

Michael