A service provider needs to implement QoS on some MLPPP interfaces on a 7500 router, but the box is operating at 70-80% of CPU utilization, this is due to some process like BGP scanner (not interruptions)
The question is: What is the recommendation about implementing QoS regarding the CPU utilization of the box?
Assuming you have CEF/dCEF enabled and recent VIPs with memory, a "simple" QoS won't do much.
But again, that will all depend on the actual QoS you are looking to apply. Don't expect to apply a hierarchical MQC with multiple classes and levels. That will probably be too much.
Use a QoS that covers your needs. Also, if you are planning in using NBAR/dNBAR, make sure you have the minimum hardware requirements for the hardware support of this feature. (VIP processor that has 64 MB of DRAM or more).
From my experience never push the 7500 to much (i've a GEIP+ - VIP4-80 with 256MB DRAM, running at 90% with around 200MB traffic), like everyone said, if you have dCEF enabled, then a simple QoS won't harm, but multiple complicated QoS policies do affect the router performance (which might not be the case for high end routers like the 7600, GSR and CRS, as they implement most of the IOS features in hardware), and please do monitor your VIP processing (to do so, please use the "if-con x" hidden command to access the desired VIP, and then do a show process cpu).
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