QoS and Router CPU Overhead

Answered Question
Feb 12th, 2008
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Hi,


Does QoS cause Router's CPU overhead?, and if it does, is it acceptable? Especially the router configured with little number of policies.


Thanks

Correct Answer by Joseph W. Doherty about 9 years 1 month ago

Both Narayan and David do well to caution about usage of NBAR, which, I think, emphasizes my point about what the QoS policy is actually doing. Some QoS features are much more resource intensive than others.


David's post, indirectly, touches on an issue which I didn't mention, it's possible multiple service interactions can especially degrade performance if the fast switching path reverts to software switching.


If both Narayan and David are implying, proceed with caution, they're certainly correct.

Correct Answer by Joseph W. Doherty about 9 years 1 month ago

"Does QoS cause Router's CPU overhead?"


Usually, some. Especially on pure routers. (L3 switches often support their QoS features within hardware.)


"if it does, is it acceptable?"


Depends much on how busy the router is before you apply the policy and what the QoS policy is doing.


"Especially the router configured with little number of policies."


Not just an issue of number of policies, but it's often more a question of what's being done within the policy or policies.


The question of the impact of QoS configuration to the router's performance is somewhat similar to performance impact questions when adding any other services feature, e.g. ACLs.


My experience has been I've rarely seen more than an additional 5% increase to the CPU load and the benefits make it worthwhile.

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Correct Answer
Joseph W. Doherty Tue, 02/12/2008 - 05:17
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"Does QoS cause Router's CPU overhead?"


Usually, some. Especially on pure routers. (L3 switches often support their QoS features within hardware.)


"if it does, is it acceptable?"


Depends much on how busy the router is before you apply the policy and what the QoS policy is doing.


"Especially the router configured with little number of policies."


Not just an issue of number of policies, but it's often more a question of what's being done within the policy or policies.


The question of the impact of QoS configuration to the router's performance is somewhat similar to performance impact questions when adding any other services feature, e.g. ACLs.


My experience has been I've rarely seen more than an additional 5% increase to the CPU load and the benefits make it worthwhile.

royalblues Tue, 02/12/2008 - 06:11
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Just to add on Joseph's post, if you are using NBAR for classifying traffic and have many traffic classes, you might see a considerable increase in the CPU


Narayan

cisco24x7 Tue, 02/12/2008 - 06:44
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This is my experience with Cisco 2621 router.

This router is running IOS 12.3(24) with

firewall feature set. It also has NAT and

CBAC. The router is connected to a cable

modem 5Mbps down and 2Mbps up. CPU is running

at 9% CPU.


When I enable QoS and NBAR on the router to

limit P2P and FTP apps, the CPU goes from

9% to 98%. Not only that, the router locks

up every 3 hours and I can no longer telnet

or ssh into the router, and that I have to

power OFF/ON the router. This happens every

3 hours.


When I disable QoS and NBAR, the issue goes

away.


You need to be careful when using these in

your production environment.


CCIE Security

Correct Answer
Joseph W. Doherty Tue, 02/12/2008 - 11:33
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Both Narayan and David do well to caution about usage of NBAR, which, I think, emphasizes my point about what the QoS policy is actually doing. Some QoS features are much more resource intensive than others.


David's post, indirectly, touches on an issue which I didn't mention, it's possible multiple service interactions can especially degrade performance if the fast switching path reverts to software switching.


If both Narayan and David are implying, proceed with caution, they're certainly correct.

habeeb_talal Tue, 02/12/2008 - 23:45
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Hi,


Thank you very much for your helpful opinions, so i'll proceed with caution and get back with the result may be it will help someone else.


Thanks

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