Always verify your router’s DRAM and Flash memory capacity before upgrading. Modern IOS images are larger than their predecessors, and you’ll want to ensure you have the overhead to support the 15.9 footprint.
Improved handling of buffers to prevent crashes during high-traffic bursts. c800universalk9mzspa1593m10bin better
Many admins running older code on C881, C891, or C819 routers report "ghost" issues—random reboots, memory leaks, or interface flaps. The 15.9(3)M10 release addresses several legacy issues: Always verify your router’s DRAM and Flash memory