On Sun, 2003-01-05 at 15:52, The Long and Winding Road wrote: > Some of us are just debating whether or not CPU utilization is the "best" > measure. Over what period? What other factors might be best brought into the > mix of factors to consider?
I'm a big fan of delay as the key performance metric for a link/router combination. In the simple case where you're just using a router to push IP packets up and down a pipe (leaving out time-sensitive stuff, QoS, etc), the real question is delay. In particular, queue depth is probably the most accurate measure of link performance. I've been hoping for better queue depth measurement tools in IOS (and other routers) for a while. There are two significant kinds of delay - transmission and queuing delay. The first one is obvious - it takes a bit X ms to get from one side of the pipe to the other. The second is a wastebasket category for everything else that causes delay, including processing time, CPU utilization, coding delay, and link utilization. You can indirectly measure queue depth by looking at ping times, but that doesn't get you directly at the real issue. You know a link is instantaneously highly utilized when it starts dropping packets off the back of the queue, etc. Also, this is complicated a bit by predictive dropping algorithms (RED, etc), and by out-of-order queuing like WFQ. There are lots of reasons that CPU is bad - think ASICs for starters, not to mention offloaded processing that happens in every variant of every major router these days. Also, re-calculating LS databases or running BGP path vectors doesn't have to impact switching performance, even though it can peg the CPU. Bits in/out isn't as much a performance metric as a utilization metric. You have no idea what the performance was like for those bits, unless you guess by inferring that the link was "pretty full", or something. MRTG-style utilization plots miss out on a lot of important detail too, because they take the bits in/out problem and make it worse by only reporting on it every 5 minutes. So, the bottom line is this: If you're seeing large queue depths and/or queue drops paired with a less-than-full link utilization over corresponding periods of time, it's time to upgrade the router. -sd Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=60445&t=60445 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
