To achieve both redundancy and load sharing between the TFTP servers, you can configure Option 150 to provide the two TFTP server addresses in reverse order for half of the DHCP scopes.
For example:
•In subnet 10.1.1.0/24: Option 150: TFTP1_P, TFTP1_S
•In subnet 10.1.2.0/24: Option 150: TFTP1_S, TFTP1_P
Under normal operations, a phone in subnet 10.1.1.0/24 will request TFTP services from TFTP1_P, while a phone in subnet 10.1.2.0/24 will request TFTP services from TFTP1_S. If TFTP1_P fails, then phones from both subnets will request TFTP services from TFTP1_S.
Load balancing avoids having a single TFTP server hot spot, where all phones from multiple clusters rely on the same server for service. TFTP load balancing is especially important when phone software loads are transferred, such as during a Cisco CallManager upgrade, because more files of larger size are being transferred, thus imposing a bigger load on the TFTP server.