To permanently enable security features on the 2951 with this image:
c2951-universalk9-mz.spa.157-3.m8.bin is more than a filename—it’s a precise specification of hardware platform, crypto capabilities, memory model, interface architecture, and software maturity. For a Cisco 2951 running in a controlled, legacy environment, it represents the last stable, fully-maintained release before the platform exited Cisco’s support lifecycle. C2951-universalk9-mz.spa.157-3.m8.bin
💡 : Use the verify /md5 flash:C2951-universalk9-mz.spa.157-3.m8.bin command on your router to check the file against the hash listed on Cisco's site. To permanently enable security features on the 2951