Hi Rich,
Here is a nice little clip on this for Cisco IP Phones;
Understanding How Devices Identify the TFTP Server
The phones have an order of preference that they use for selecting the address of the TFTP (Trivial File Transfer Protocol) server. If the devices receive conflicting or confusing information from the DHCP server, the device uses the following sequence to determine what information is valid:
1. You can locally configure the phone with a TFTP server. This address overrides any TFTP address sent by the DHCP server. The phone always tries to resolve the DNS name CiscoCM1.
2. If this name is resolved, then it overrides all information sent by the DHCP server.
It is not necessary to name the TFTP server CiscoCM1, but you must enter a DNS CName record to associate CiscoCM1 with the address or name of the TFTP server.
3. The phone uses the value of Next-Server in the boot processes. This DHCP configuration parameter has traditionally been used as the address of the TFTP server. When configuring BOOTP servers, this field is typically referred to as the address of the TFTP server.
This information is returned in the siaddr field of the DHCP header. You should always use this option, if available, because some DHCP servers will place their own IP address in this field when it is not configured.
4. The phone uses the site-specific option 150.
5. The phone also accepts the Optional Server Name parameter. This DHCP configuration parameter is the DNS name of a TFTP server. Currently only a DNS name can be configured in this parameter; a dotted decimal IP address should not be used.
6. The phone also accepts the 66 option, which is the name of the boot server.
7. Option 66 is normally used to replace the sname field when option overloading occurs. It can be used on Windows NT DHCP servers and functions like the 150 option. This name field can contain a DNS name or a dotted decimal IP address.
8. The 66 option should not be used with the 150 option. If they are sent together, then the phone prefers the IP address over the name given by the 66 option. However, if both a dotted decimal IP address and a 150 option are sent, then order of preference is dependent on the order that they appear in the option list. The phone chooses the last item in the option list. To reiterate, option 66 and option 150 are mutually exclusive.
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_administration_guide_chapter09186a0080532bf8.html
Hope this helps!
Rob
