Hopefully you have found a more comprehensive answer to your question by now. If not, the C881-K9 is the second generation of the 800 Series routers. As compared to the first generation (Cisco 881), the C881 has a much more powerful processor and will handle encryption much, much better. I used a first-generation router to temporarily replace a second-generation router on an IPSec tunnel. The first generation router's CPU was running in the 80 to 90 percent utilization and affecting response times. The second generation router's CPU runs at under 10% utilization.
Just trying to clear up some uncertainty on this.
Have a device which I was told was to be replaced with the currenr gen. C881.
But a show inv on the current one looks like I already have up to date hardware :-
NAME: "C881-K9", DESCR: "C881-K9 chassis, Hw Serial#: FCZ1853xxxx, Hw Revision: 1.0"
PID: C881-K9 , VID: V01, SN: FCZ1853xxxx
and from sh ver :-
Cisco C881-K9 (revision 1.0) with 488524K/35763K bytes of memory.
250880K bytes of ATA System CompactFlash (Read/Write)
*0 C881-K9 FCZ1853C1BM
What would the EoL 1st gen 881 ISR show as the model under it's show ver / show inv ?