write memory - CPU Utilization

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Sep 5th, 2008
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I have a 1841 running on c1841-spservicesk9-mz.124-7d.


I see more than 80% of CPU utilization soon after saving the configuration(write mem), which lasts for several seconds. Could this lead to a router crash at a particular stage?


Is it due to a bug in IOS?


Correct Answer by Giuseppe Larosa about 8 years 10 months ago

Hello Ranil,

the cpu on a router like yours can have these spikes.


About why saving the config can cause a so high cpu usage I think that modern routers that are SDM ready can actually have to create an XML file for the running config, then they have to translate it to a simple text file and to write it on the NVRAM.

So the duty can be now heavier then in the past.


I usually look at the cpu average 1min as the parameter to decide if a router is working fine or it is overloaded.



Hope to help

Giuseppe


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Collin Clark Fri, 09/05/2008 - 13:30
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Router crashes are due to software bugs, not hardware utilization. The 1800's are small routers and use a lower end processor. My 1800's do the same and I have never had a problem with them yet.


Hope that helps.

Giuseppe Larosa Fri, 09/05/2008 - 13:31
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Hello Ranil,

if the cpu goes over 80% only for several seconds it shouldn't lead to a router crash.


Is it due to a bug in IOS?


It may be after having seen a bug about HSRP in your other post ...:)



Hope to help

Giuseppe



rsgamage1 Fri, 09/05/2008 - 14:17
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Hi Giuseppe,


You are quite right. In fact, I came across another bug which says router crashes when the write memory and

secure boot-image commands are executed simultaneously(Ref: CSCsg69124).


I've been thinking since about a week regarding this high CPU utilization when saving the runnging config.


When I noticed that it consumes lot of processing power, I was a bit suspicious that this symptom also has something to do with the affected image.


So do you mean to say that this normal and expected?


FYI: the average CPU utilization is around 2% at the moment.

Joseph W. Doherty Fri, 09/05/2008 - 16:19
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It's likely normal behavior, since many Cisco software based routers spike their CPU when writing to the flash, such as loading a new IOS via tftp or FTP. Plus, the CPU might have to parse out the config before it saves it (often there's a delay before a show run vs. a show config.)

Correct Answer
Giuseppe Larosa Sat, 09/06/2008 - 01:27
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Hello Ranil,

the cpu on a router like yours can have these spikes.


About why saving the config can cause a so high cpu usage I think that modern routers that are SDM ready can actually have to create an XML file for the running config, then they have to translate it to a simple text file and to write it on the NVRAM.

So the duty can be now heavier then in the past.


I usually look at the cpu average 1min as the parameter to decide if a router is working fine or it is overloaded.



Hope to help

Giuseppe


rsgamage1 Mon, 09/08/2008 - 08:00
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Hi,


Many thanks for sharing your experiences and thoughts . I'm convinced that this is expected for an SDM enabled low-end ISR.


I've made an IOS upgrade as well(to correct the HSRP bug); CPU utilization remains the same as it was before.


Regards,


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