CPU Load 98% after changed MTU size

Question

Hi,
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

Answer 1

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.