IOS version 15 is the final major release for the 3900 series (support ended in 2020 for most models). Version 15.7(3)M8 is a , meaning it prioritizes bug fixes over new features.
This image leverages the onboard to offload encryption and compression tasks. Without this specific image, advanced features like Zone-Based Firewall (ZBFW) and Cisco Unified Communications Manager Express (CUCME) will not function correctly on these platforms. C3900-universalk9-mz.spa.157-3.m8.bin
license install flash:FLASEC-K9-3945.lic IOS version 15 is the final major release
However, this image is suitable for: