Question : Cisco 3640 high CPU at 5kpps with CEF?

I have a 3640 running 12.2(11)T9 - latest IOS that will fit into 16Mb flash.  It is using IP/IPX/AT Plus, and has 96Mb dram.  With only two FE interfaces active, it reaches 80% CPU with around 5000 TCP packets per second (140 byte packets).  I am using it to route some circuit-emulation traffic between two RAD IPMux boxes.  If I remove CEF and use normal fast-switching, the CPU is about the same.  Adding netflow takes a little more CPU but not much.  Utilisation is typicaly 80%/78%, so almost everything is being fast switched, as the "show int switching" command shows.  If I use UDP, the CPU maxes out at around 3800 pps (140 bytes).  I have stripped the config down to the bare minimum and removed other interfaces with zero effect.  I have removed IPX and the G.SHDSL WICs and even the NM-2Ws and the second NM-2FE2W.  I've tried it between two interfaces on the same NM-2FE2W and between two separate NMs in two different slots, in 0 and 1 and 0 and 2 with zero effect also.

I have a 3745 at the other end of the link that runs this traffic at less than 6% CPU.  I have removed the LLQ/CBWFQ and set no fair-queue, with minimal effect.

I thought the 3640 could do at least 30k packets per second fast-switched, am I missing something here?

Answer : Cisco 3640 high CPU at 5kpps with CEF?

My experience with small packet traffic is exactly what you are seeing, although your pps rate seems low for the high cpu utilization.

Some possibilities to improve the response is to use 2 single port FE interfaces which seem to use less cpu resources than 1x 2 port interface, or using 2x 2 port FE interfaces.

Small packets are a killer on the older gear, but the newer Cisco equipment, 3550 for example, runs about 3% utilization doing the same thing.