We have a Cisco ISR 3945E that came with 1GB (512MBx2) We need to bump up to 2GB so we purchased through our channel a 2GB module part# MEM-3900-2GB=. What we noticed is that the memory needs to be installed in pairs. the 2x 512MB work fine, but when we install the 2GB module in SLOT 0 or SLOT 1 we get a memory error. But when we install 2x2GB modules the router comes up fine. Can anyone validate that memory on the ISR 3945e must be replaced in equal memory pairs?
That is correct, both DIMS have to be populated equally:
Form the config guide:
Both the Cisco 3925 and Cisco 3945 ship with a default memory of 1 GB DRAM and 256 MB Compact Flash. The 1 GB DRAM configuration is derived by the installation of 2 symmetrical DIMMs of 512 MB in each of the memory slots on the SPE of the Cisco 3900 Series routers. While upgrading memory to higher densities, it is necessary that both the DIMMs are populated with DIMMs of symmetrical type and density for high system performance. The Cisco 3900 Series does not support the use of different DRAM densities in the two available DIMM slots or the use of only one slot. The memory type and SKUs are shared between Cisco 3925 and Cisco 3945 as listed in Table 3.
Here is the link to it: