TFTP on 7936 and 7910 will not release

Tommer Catlin · ‎06-05-2009

Anyone know why after we changed our TFTP server on our DHCP scopes, our 7940/7960 phones rebooted, got the new scope, but our 7910 and 7936 will not??? Tried factory resets, etc. Plugged a 7940 into the same port, it got the tftp... so I thin it's phone related.

Tommer Catlin · ‎06-05-2009

Ok, so here is the deal. Apparently, I had the perfect storm tonight.

Old Publisher name is ciscoum1. Which or course, built into the firmware files of the older phones 7910 and 7936 DEFAULTS to this *if* you have DNS configured in your DHCP scope and *overides* the TFTP option 150. buggers... Sniffers are quite handy I must say. And really old docs...

Understanding How Devices Identify the TFTP Server

The phones and gateways have an order of preference that they use for selecting the address of the TFTP 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 (but not a gateway) 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.

This option resolves the issue that Microsoft Windows NT servers do not allow the Next-Server configuration parameter. The Microsoft Windows NT servers allow access to the Next-Server parameter only when IP address are statically assigned.

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.