I have 2 CCM 4.2 and i have run on both TFTP server and instructed the phones to have an alternate TFTP through DHCP.
i did the failover where i have stopped the TFTP server on the primary and the DHCP on the primary and get a new IP with a TFTP server of the subscriber. it worked fine!!!
tomorrow i will test extension mobility to see how it will react (i read it earlier that it will not).
But my question is, is it right what i have done, am i missing something.