CPU Load 98% after changed MTU size

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Mar 6th, 2009
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I am having problem here. Previously, when the MTU was 1508...The cpu load was <5%. When changed to 1512, the load was increasing 70%. Recently, changed to 1522, the load is >95%. Not sure what's going with the configuration. But I can a lot ques dropping. Should I put on the port-channel 1? For more details configuration, please refer to attached file

hold-queue 1024 in

hold-queue 1024 out

# show proce cpu

CPU utilization for five seconds: 96%/38%; one minute: 96%; five minutes: 96%

PID Runtime(ms) Invoked uSecs 5Sec 1Min 5Min TTY Process

# show proce cpu | include Tag

165 150300 11396947 13 0.07% 0.01% 0.00% 0 Tag Control

193 2608887200 237733062 10974 52.23% 52.71% 52.44% 0 Tag Input

304 4692 36566 128 0.00% 0.00% 0.00% 0 Tagcon Addr

Any idea guys. Thanks lot

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shivlu jain Fri, 03/06/2009 - 02:53
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could you tell me which router and ios you are using. Are you using sub interfaces on the router and mpls on router.


shivlu jain

lawwm Fri, 03/06/2009 - 07:45
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cisco CISCO7609 (R7000) processor (revision 1.2) with 983008K/65536K bytes of memory.

ROM: System Bootstrap, Version 12.2(17r)S4, RELEASE SOFTWARE (fc1)

BOOTLDR: s72033_rp Software (s72033_rp-ADVIPSERVICESK9_WAN-M), Version 12.2(18)SXE5, RELEASE SOFTWARE (fc1)

System image file is "disk0:s72033-advipservicesk9_wan-mz.122-18.SXE5.bin"

2 physical Ge interfaces bundle to port-channel.

If not mistaken, by increasing mtu size shouldn't increase the load.

You can refer configure attached.


autobot130 Fri, 03/06/2009 - 08:31
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I had this same exact problem on the cat 6500 running SXF10. Where I enabled MPLS and it caused CPU to spike. So to check what is getting forwarded to the MSFC perform the following to see a packet capture of what is hitting the CPU. So you need to run a SNIFFER capture.

Note this is only for the 6500 platform... once you check what's hitting the CPU (eg: fragmentation, icmp type 3 code 4's, etc al)...

Here are the instructions to setup inband span (which monitors traffic sent

to the MSFC):

Router#monitor session 1 source interface fa 3/3 !--- Use any interface that

is administratively shut down.

Router#monitor session 1 destination interface fa 3/2 !-- connected to sniffer port

Now, go to the SP console. Here is an example:

Router#remote login switch

Router-sp#test monitor add 1 rp-inband rx <--- check the syntax as it varies

from one IOS to the next so use ?

Verify monitor session:

Router-sp#test monitor show session 1

Ingress Source Ports: 3/3 15/1

Egress Source Ports: 3/3

Ingress Source Vlans:

Egress Source Vlans:

Filter Vlans:

Destination Ports: 3/2

Go back to the RP and verify the monitor session as well:

Router#show monitor

Session 1


Type : Local Session

Source Ports :

Both : Fa3/3

Destination Ports : Fa3/2

SP console:

Router-sp#test monitor session 1 show

Ingress Source Ports: 3/3 15/1

Egress Source Ports: 3/3

Ingress Source Vlans:

Egress Source Vlans:

Filter Vlans:

Destination Ports: 3/2

To remove the inband span from sp do test monitor session 1 del and from the rp do no mon sess all


So my exact issue was two parts, CPU spike up 99% when WCCP was enabled with MPLS tag switching at the same time. Rebooting didnt help... I ended up changing from SXF10 to SXF14 IOS just in case but I still had high CPU after upgrade. But the major difference was when I removed the ACL tied to WCCP and removing the ACL completely and re-pasting into the configs for WAAS and then reapplying. But in my sniffer capture I did notice a lot of ip fragmentation hitting the MSFC processor. Now my MTU is 1522 and works fine.

lawwm Fri, 03/06/2009 - 17:35
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I really appreciated your solution. I will try it out. Actually my scenario is

same location (1 year)



due to additional traffic, the traffic hit 1.2G caused some of packet dropped

I was forced to bundle 2GE from(by vendor)

7609<-->12140 with 1508


All the while, it's fine. Until my users had file complaints that some emails attachment cannot go thru.

Even it's locate LAN, MPLS is enabled. I calc. 8 extra headers are needed. I tried to add a bit 1512 and test with users(improvement). The CPU load was >70%. I tried add 1522, whoo 98%.

I think part of the solution is to sniff it. It's remote site, I will try to do it.

Anyone of you encounter?

Thanks a lot autobo


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