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How does an IP phone find it's Call Manager?

I have only recently got into IP Telephony and wondered how an IP Phone is able to locate it's call managers when simply plugged into a switch port?

I know the phone pick ups its IP address/mask/gateway and dns via a DHCP server much like a PC and know the call manager provides all other relevant information like giving the phone a number but I never understood how the IP phone picked up the Call Manager's Ip address, especially when the call Manager is far away over a routed link?

I am sure it is something obvious but any answer is much appreciated.


Re: How does an IP phone find it's Call Manager?

When the phone boots, it gets it's IP address from the DHCP server, and in addition to that, the DHCP server provides another setting: TFTP server. The phone then contacts the TFTP server and downloads a configuration file specific to it (based on device name) which identifies where it's primary, secondary, tertiary CallManagers are located, among other settings.

The files on the TFTP server are maintained by the CallManager (and the TFTP server itself is hosted on one of the CallManagers). By default, CallManager doesn't generate actual files on the TFTP server like it used to in the past. Instead, it provides TFTP files dynamically as requested by the phones.

This document might be useful in explaining this process:

In addition, these pages might be useful:

Installing the Cisco IP Phone: Verifying the Phone Startup Process

Cisco Unified IP Phone Administration Guides for Cisco Unified CallManager Models 7960, 7940 and 7910 (SCCP)

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