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High CPU usage caused by interrupts


Dear community.

After using my cisco 2621 as my internet gateway, and upgrading my DSL connection to 20mbit/1mbit, my cpu usage has increased to almost 90%. These 90% are reached when i am downloading @ 15mbit/sec.

After issueing a 'show proc cpu sorted' command, the 80% cpu usage was not caused by any process, but by interrupts:

CPU utilization for five seconds: 87%/81%; one minute: 1%; five minutes: 3%

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

41 68700 76661 896 0.95% 0.19% 0.16% 0 IP Input

I am already using 'ip cef' on both the FA interfaces. And issueing a 'sh int stat' command result in a very healthy routing proces if you ask me, if you look at the cache/proces ratio:


Switching path Pkts In Chars In Pkts Out Chars Out

Processor 75394 6417511 46889 5303988

Route cache 415701 36392514 645159 763558263

Total 491095 42810025 692048 768862251


Switching path Pkts In Chars In Pkts Out Chars Out

Processor 16157 3375901 27213 2645817

Route cache 639952 764440471 415434 37734183

Total 656109 767816372 442647 40380000

NIDWORKR01#sh ip cef sum

IP CEF with switching (Table Version 116), flags=0x0

94 routes, 0 reresolve, 0 unresolved (0 old, 0 new), peak 0

94 leaves, 127 nodes, 144864 bytes, 119 inserts, 25 invalidations

0 load sharing elements, 0 bytes, 0 references

universal per-destination load sharing algorithm, id 3EDCA274

3(0) CEF resets, 1 revisions of existing leaves

Resolution Timer: Exponential (currently 1s, peak 1s)

1 in-place/0 aborted modifications

refcounts: 32828 leaf, 32768 node

Table epoch: 0 (94 entries at this epoch)

Adjacency Table has 90 adjacencies

81 IPv4 adjacencies

I have already followed the cisco doc which mentions the 'high cpu usage caused by interrupts'. These steps did not lead to decrease my cpu usage. Can anybody point me in a direction to troubleshoot the traffic that is causing this high cpu usage? Could this problem be CEF related?

Btw, IOS version 12.3(21)

Thanks in advance!


Danilo Dy

Can you post the output of "show proc cpu" or "show tech-support"?

Hi Medan,

Here is my 'show proc cpu' when downloading @ 15mbit:

NIDWORKR01#sh proc cpu | ex 0.0

CPU utilization for five seconds: 93%/81%; one minute: 24%; five minutes: 13%

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

3 20036 303 66125 10.95% 4.83% 2.44% 66 SSH Process

41 2480 1368 1812 0.55% 0.32% 0.22% 0 IP Input

I already tried upgrading to the latest 12.3(22) but no luck either. I think it's a bug somewhere but i am not sure at this. I hope someone can point me in a direction to let me show what interrupts are causing the cpu to go through the roof

I have included "show tech-support" in this one.


Your high CPU utilization is cause by SSH not .

PID CPU Time Process

3 28.69 SSH Process

Try the following and use telnet unless you are connecting from unsecured network (internet)


line vty 0 4

transport input telnet ssh

Improve TCP performance


service nagle

service tcp keepalive-in

service tcp keepalive-out

Hi medan,

SSH indeed caused some cpu spikes, but this was caused by my session to provide the output of "show tech-support". After that i started a downstream of 15mbit, and logged out of my SSH session. After a while i checked my cacti graph, and it was still peaking @ 75% cpu usage.

I will try to enable the services you mentioned above to improve tcp performance.

Thanks already!