[c-nsp] how "fast" is L2TP "fast switching" under 12.3T?

[Robert E. Seastrom]

So...  I have a DSL wholesaler providing me L2TP handoff (slight
variant on the classic LAC to LNS VPDN model:  customer's PPPoE
translated to L2TP by the wholesaler, handed off on a .1q trunked
ethernet, with one or more LACs on each trunk).

I'm running c7200-js-mz.123-8.T4.bin on a 7206 VXR with an NPE300
(PA-FE-TX facing the wholesaler in slot 2, IO-FE talking to the core)
as the LNS, and noticed that my CPU seemed abnormally high for the
amount of traffic I'm moving, compared to a friend's VXR running
12.3(5a) mainline with ATM handoff to another wholesaler.

A bit of investigation showed that 12-14% of the CPU (overall CPU
utilization 40-45% with 15 mbit/sec towards the customers and 4
mbit/sec from the customers) was spent in "IP Input".  A capture of
packets to the log buffer by "debug IP packet" showed incoming packets
(from the LAC to the LNS) as "routed via RIB" whereas outgoing packets
over the virtual-access interface showed as "routed via FIB".
Everything else looks nominal, but "routed via RIB" sounds an awful
lot like process switching to me.

I've gone over the config and not seen anything obvious that would be
making this happen - is there perhaps a better choice?  should i be
moving to 12.3 mainline and get off the T train?

Thoughts?

                                        ---Rob