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

Krzysztof Adamski k at adamski.org
Tue Oct 26 23:43:05 EDT 2004

Your problem is fragmentation, since the L2TP tunnel is delivered over
Ethernet, the max MTU is 1500. Every customer frame that is over 1452
causes fragmentation since 1452 + 8(PPPoE) + 12(L2TP) + 8(UDP) + 20(IP) = 1500.
You can add "ip tcp adjust-mss 1412" to eliminate fragmentation of TCP
packets, but you won't be able to do anything about UDP and IPSec packets.

You friend has ATM, he has a bigger MTU, so no fragmentation.

I have an identical setup to you, I graph CPU usage and reassembly, the
graphs match perfectly.


 On Tue, 26 Oct 2004, Robert E. Seastrom wrote:

> 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
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