[nsp] l2tp-cpu load

Andy Furnell andy at furnell.org.uk
Fri Nov 28 07:27:31 EST 2003

On Fri, Nov 28, 2003 at 11:05:26AM +0000, H S wrote:
> Hi !!
>        I have a Cisco 3640 and have set up an l2tp tunnel. I wonder how 
> many simultaneous sessions will it support. Currently, I have an average of 
> 16 sessions and 26 % of CPU load. The traffic through the interface is not 
> much (1 Mbps) and (400 pps) I?m planning to change the 3640 by a 7204 
> (NPE-400) but I would like to know it?s limitation on this issue.
> Could anyone advise me?
> Thanks in advance!!!!
> Regards
> Hugo

Hi Hugo,

I had an NPE-400 terminating ~1500 l2tp dial sessions at around 45% CPU... 
This box was also handling a couple of PA-MC-E3 and a PA-MC-STM1 with a total
traffic load of about 35mbit/sec and ~15kpps. This took some messing around
to get CPU usage below 60%... PPP compression is right out the window,
and vpdn ignore udp checksum in your global config will reduce CPU usage by
another 10-15%. The only problems we ran into were with virtual-access
interfaces not being re-used - the only solution for this at the time was
to reload the machine every month, but this bug may well be fixed in a more
recent IOS release. As far as IOS releases go, we had the best luck with 
12.2(8)T, although I've heard good things about 11T and 15T, as well as 12.3

Remember the number of tunnels will affect performance much more than the 
number of sessions... if you can aggregate the l2tp tunnels so only one tunnel
is presented to your LNS you'll get much better performance than if you're 
forming hundreds of l2tp tunnels to hundreds of LACs with a similar number
of active sessions.

If the cash is there you'll have get much better performance from an NPE-G1.
>From speaking to people using them as a broadband LNS they seem to be able
to take an STM-1's worth of traffic in about 5500 sessions without breaking
a sweat :)


Andy Furnell
andy at furnell.org.uk

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