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QoS and Router CPU Overhead

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

2 ACCEPTED SOLUTIONS

Accepted Solutions
Super Bronze

Re: QoS and Router CPU Overhead

"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.

Super Bronze

Re: QoS and Router CPU Overhead

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.

5 REPLIES
Super Bronze

Re: QoS and Router CPU Overhead

"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.

Re: QoS and Router CPU Overhead

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

Silver

Re: QoS and Router CPU Overhead

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

Super Bronze

Re: QoS and Router CPU Overhead

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.

New Member

Re: QoS and Router CPU Overhead

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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