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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!

16 Replies 16

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

If you're pushing 15 Mbps through a 2621, your CPU performance is within expectations.

A chart that's been reference within these forums,, show a 262x can do about 13 Mbps using fast/cef switching.

Ah. Thanks for sharing this chart.

Still though, I am wondering what kind of hardware routers ISP's are using these days for the SOHO market. The standard router my ISP delivers on a 20mbit line, is a SMC 79xx router. I can't imagine these routers have better hardware built-in as Cisco does?

But, regarding this pdf, the cpu is indeed within expected ranges.

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