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High CPU load on msfc sup720 while using nat pool

Unanswered Question
Jun 5th, 2008
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On our 6509-E+switchblades with sup720/pfc3 and CSM module we noticed a considerable cpu load like:

#show processes cpu sorted

CPU utilization for five seconds: 85%/81%; one minute: 82%; five minutes: 41%

after some research i'm able to reproduce it, and basically its:

when sending traffic through the vlans defined on the msfc with nat inside and nat outside it's reproducable.

when unconfiguring NAT the cpu load drops (in lab) to 0%/0%.

we're using nat pools just to fix a internal application/service on 1 IP.

it's configured like:

ip nat pool DMZ-193 netmask

ip nat inside source list DMZ-193 pool DMZ-193 overload

ip access-list extended DMZ-193


where the the external (example) source IP is where it's S-natted to.

With this "feature" i can't get a higher rate then about 130Mbit/s (msfc cpu bound)

Has any one an idea why this gets executed in software and not in hardware like what the docu says?

Any idea or workaround is welcome.

additional note: i reviewed document:


which gave good ideas, but no solution yet.


Arjan Filius

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siennax Tue, 06/10/2008 - 05:29
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problem solved, there where some empty ACL's which causes to use the cpu instead of hardware.



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