[nsp] MPPP vs CEF

[jlewis at lewis.org]

Today I tried converting a 6xT1 connection between a 7206 and 3640 from
CEF load sharing to multilink PPP.  The routers involved are:

7206 NPE200 12.2(14)S3 (I doubt this end is relevant)
3640 12.2(17a) NM-1FE, 2x NM-4T

The 3640 only has the 6 T1's to the 7206 (2 ports currently unused) and a 
the FE feeding the LAN.  The 3640's FE port typically peaks at about 
6mbit/s out, 2mbit/s in.  CPU load ranges from about 15% middle of the 
night, 50-85% during the day/evening.  CEF is enabled on all 
interfaces.  

We've gotten some complaints about VOIP jitter, so first I switched the
T1's from per packet to per destination load sharing.  The for some
reason, I decided to give multilink PPP a try.  As soon as I converted the 
T1's to encaps PPP and put them in multilink-group 1, CPU load shot up 
from 50% to 94%.  Over the next hour, the CPU load rose to about 99% at 
which point the 3640's CLI became rather unresponsive and pings to the 
3640 varied from sub-100ms, to as bad as 50000ms!

Did I run into a bug in 12.2(17a) (this same version had serious CPU load 
issues on some of our other 3640's) or is multilink still expected to eat 
lots of CPU even in versions where it's CEF switched?  While the load was 
95-99%, show proc cpu didn't show any processes actually using any 
noticable amounts of CPU time.

For now, it's back to CEF per destination load sharing.

FWIW, one other serious downside I noticed with multilink was any changes 
to the multilink interface (including adding/removing members) seemed to 
stop the flow of traffic momentarily (I remember this being a problem when 
I used to use ISDN and the second B channel would come up on demand) and 
doing something like "no cdp enable" would cause the multilink interface 
to flap.
 
----------------------------------------------------------------------
 Jon Lewis *jlewis at lewis.org*|  I route
 System Administrator        |  therefore you are
 Atlantic Net                |  
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