This document is based on a common customer issue based on actual Cisco
TAC Service Requests (SR).
This document's scenario starts with a B250M2 populated with 24 X 4G
DIMMs running on ESXi.
Next, more memory needs to be added to the server. 24 X 8G DIMMs are
ordered. The 8G DIMMs are then inserted in the available slots.
Now when it boots the server will not have "checking memory". If DIMM
locations are swapped the result is the same. It only seems to work when all 4G
or all 8G DIMMs are installed.
Here are the assumptions in this example:
There are two identical CPUs.
All DIMMs are the same speed.
All the DIMMs installed have been verified that they are supported
for this server and release.
All DIMMs are DUAL Rank DIMMs.
Technical Tips Conventions for more information on document
How do I get DIMMs of different sizes (4G and 8G) to work in the same
A. First, you need to understand the CPU to DIMM channel relationship as
Here are the DIMM population rules per channel:
According to the
Spec sheet the B250 supports a total of 48 DIMM slots.
Here are some key points from the above document:
System performance is optimized when the DIMM memory type and the DIMM
quantity is equal for both CPUs. Performance degradation can result from the
Only Cisco memory is supported. Third party DIMMs are not tested or
All DIMMs within the B250 M2 server should use the same clock
frequencies. Mixing clock frequencies is not supported.
If the system has two CPUs, the DIMM slots for both CPUs should be
populated in an identical manner.
Populate DIMMs of different size and organization in separate memory
channels. Populating different sized DIMMs within a channel is not supported.
For example you cannot put single-rank 4GB DIMMs in the same channel as
dual-rank 4GB DIMMs.
The B250 M2 server needs at least one DIMM pair installed for CPU 1
or CPU 2.
Carefully match CPU and DIMM speed. If the CPU and DIMM speeds do not
match, the system runs at the slower of the two speeds.
B250 M2 server memory is always sold as a correctly matched pair with
identical manufacturer, type, speed, and size, intended to be installed
together in the two paired banks of a single UCS Server memory channel. The
mixing of unpaired DIMMs (even with other DIMMs sold under the same product ID)
results in memory errors if a mismatch occurs. When you install DIMMs in a B250
M2, add matched pairs to the channel slots in the order shown in the tables of
this section. This server does not support odd numbers of DIMMs in a channel,
or a configuration of 6 DIMMs per channel.
These steps need to be completed:
Do NOT mix DIMM densities within a pair (No 4G and 8G DIMMS in the
Populate the DIMM slots the same for both CPUs.
Populate only like DIMMs in the same channel (No 4G and 8G DIMMS in
the same channel).
Make sure the there is an even number of DIMMs in the
Only 2, 4, or 8 DIMMs are supported per channel.
DIMMS are NOT supported on this
What there is in the mock customer environment:
24 * 8G DIMM = 192G
24 * 4G DIMM = 96G
Recommended B250 M2 Memory Configurations per CPU:
8G X 8 (A0,A1) (A4,A5) (A2,A3) and (A6,A7)
4G X 8 (B0,B1) (B4,B5) (B2,B3) and (B6,B7) 8G X 8 (C0,C1) (C4,C5)
(C2,C3) and (C6,C7)
Note: CPU is populated the same.
Boot the server and all is well.