3750 High CPU Utilization

Unanswered Question
Nov 2nd, 2008

I have a 3750 with IOS Version 12.2(25)SEE3. Once or twice a day i have alarms on High CPU utilization on the switch. "HL3U bkgrd process" , is the process which is eating up my CPU. I try to Google but cant find the details of this process anywhere. Has anybody some info about this process ??

Further, during the search i read this on cisco.com, it says : "A capacity issue is one of the symptoms of high CPU utilization on Cisco IOS routers. However, a capacity issue is almost never a symptom of high CPU utilization with hardware-based forwarding switches." Please elaborate it.

Thanks all.

I have this problem too.
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satish_zanjurne Sun, 11/02/2008 - 22:29

1.My perception of Capacity here is backplane capacity of the device.

2.In ths case of Catalyst 3750 it is 32Gbps

3.As it is L3 switching or hardware based forwarding , it should not be issue for Catalyst 3750

4.As in router it is switching process based for it , that was the issue.

HTH...rate if helpful..

Joseph W. Doherty Mon, 11/03/2008 - 17:03

To obtain further information on about the "HL3U bkgrd process", assuming another poster doesn't provide it, might require TAC. Or, the "Output Interpreter" tool might shed some additional light on it.

Something you might try, moving to a more recent IOS. Either 12.2(25)SEE4 or another later/higher SE release.

With regard to the capacity difference between many routers and hardware based switches, the former (i.e. software based routers, like the ISRs) rely on the main CPU. The latter, like the 3750, have ASICs (application specific integrated circuit), i.e. dedicated hardware, for normal packet forwarding.

As an example, the 7200's NSE-G2 (very high end for a software router) is rated at 2 Mpps, while the 3750 series tops out at about 38.7 Mpps. The former will show 100% busy CPU, but unsure what the latter will show for its CPU when totally saturated. (It might require a different show command, such as show controller[?], to reveal how busy the 3750 hardware is.)

One thing to note, both routers and switches have been optimized for their primary function but some types of traffic processed by either, especially switch traffic not processed by ASICs, will very much impact the device's CPU.

krunal_shah Mon, 11/17/2008 - 12:04

high cpu with ARP input and HL3U bkgrd process


Cisco Catalyst 3750 and 3560 switches will show high CPU utilization in the HL3U

process when running Cisco IOS 12.2(18)SE through 12.2(20)SE2 with a large number

of proxy ARP entries. In some rare conditions, the switch may reload.


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