Dell R710 RAM

Hi All,

I have a Dell R710 server. I have populated all 18 slots with 4Gb RAM modules. The BIOS is showing 72GB if RAM at 800Mhz.
During Boot I get  the following Error/Warning

Unsupported memory config. DIMM mismatch across slots detected.
The following DIMMS should match in geometry DIMM A4, A5, A6
The following DIMMS should match in geometry DIMM B4, B5, B6

I'm not sure why I am getting the error.
I have read this:

Taken from here: ftp://ftp.dell.com/Manuals/all-products/esuprt_ser_stor_net/esuprt_poweredge/poweredge-r710_User's%20Guide7_en-us.pdf

The memory speed of each channel depends on the memory configuration:
• For single or dual-rank memory modules:
– One and two memory modules per channel support up to 1333 MHz.
– Three memory modules per channel support up to 800 MHz.
• For quad-rank memory modules:
– One memory module per channel supports up to 1067 MHz.
– Two memory modules per channel are limited to 800 MHz, regardless of
memory module speed.

is this error because I am using 3 modules per channel and the speed has dropped to 800Mhz.
The bios is also running in optimizer mode and I cant change this mode (it is fixed)

Thanks in advance.