Older images like 15.6M, 15.7M, or early 15.8M trains are susceptible to critical operational bugs that M10 definitively addresses. Feature Category Legacy 15.x Images The 15.9(3)M10 Standard
spa : This could denote a specific hardware platform or additional features related to the SPA (Service and Application Module) or possibly the packaging or distribution method. c800universalk9mzspa1593m10bin better
Cisco IOS version 15.9(3)M10 c800-universalk9-mz.SPA.159-3.M10.bin Older images like 15
Some older 800 series bootloaders (ROMmon) expect the older sp naming convention. If your router has an ancient bootloader, trying to load spa1593m10 might result in a loadprog: bad file magic number error. In that case, you need to upgrade the bootloader first, making the process harder than using a legacy image. Older images like 15.6M