2811 CPU and a 50Mbit pipe

Answered Question
Nov 4th, 2009

Hey all!

My scenario is as follows:

We have a 2811 Edge router that has traffic-shaping policies (4 incoming, 4 outgoing) 1 server that pulls snmp information and 2 systems that pull flow-cache information on demand.

Available bandwidth is low, hence the shaping. We want to up the bandwidth to 50Mbit from 20Mbit. Cisco states the device can only handle 61 Mbit. Our provider states that 100% CPU utilization (and subsequent packet loss) was reached at 52Mbit.

At 20Mbit, our CPU utilization is 56%.

So. Two questions:

1. At one point does CPU utilization become problematic? Is there a threshold that Cisco recommends a router not hit? I'm having trouble slogging through the available online information, and the answer eludes me.

2. What's taking up the majority of the CPU usage? Due to the low utilization of all other processes (the highest is 3.11% on any one process), I have to assume that it's traffic. (Attached is the CPU cap).

Let me know if you require additional information.

Thanks!

Grant

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Correct Answer by Joseph W. Doherty about 7 years 1 month ago

At 99%, router might not keep up with offered load, so you want to avoid that. As you approach 100%, there's little CPU cushion for other things, so you want some head room. I think somewhere I saw Cisco recommend to avoid going past 80% load for routine operations. A 20% cushion probably is okay for a minimum cushion.

For 50 Mbps Ethernet(?), you would probably be okay with a 3825 and might be okay with a 2851. I know from experience a 2811 goes flat out on a full DS3 (45 Mbps) and that 3825s seem to usually be okay with them.

Attached is a reference document for router performance.

Correct Answer by Paolo Bevilacqua about 7 years 1 month ago

Even without going into details, a 2811 for 50 Mbps appears to be undersized.

Remember, when cisco says e.g. 61 Mbps, they mean unidirectional traffic under most favorable conditions (larger packet size and no features enabled). So that would translate to 30 Mbps of traffic active in each direction. That pretty much matches what your SP is telling you.

Seems to me you have to get a bigger router without any doubt.

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Correct Answer
Paolo Bevilacqua Wed, 11/04/2009 - 12:38

Even without going into details, a 2811 for 50 Mbps appears to be undersized.

Remember, when cisco says e.g. 61 Mbps, they mean unidirectional traffic under most favorable conditions (larger packet size and no features enabled). So that would translate to 30 Mbps of traffic active in each direction. That pretty much matches what your SP is telling you.

Seems to me you have to get a bigger router without any doubt.

grobinson23 Wed, 11/04/2009 - 13:13

Great! This is excellent information. Is there a resource that I could go to on CPU utilization? At what point do packets begin to drop out? 61%? 80%? 99%?

Thanks!

Correct Answer
Joseph W. Doherty Wed, 11/04/2009 - 13:29

At 99%, router might not keep up with offered load, so you want to avoid that. As you approach 100%, there's little CPU cushion for other things, so you want some head room. I think somewhere I saw Cisco recommend to avoid going past 80% load for routine operations. A 20% cushion probably is okay for a minimum cushion.

For 50 Mbps Ethernet(?), you would probably be okay with a 3825 and might be okay with a 2851. I know from experience a 2811 goes flat out on a full DS3 (45 Mbps) and that 3825s seem to usually be okay with them.

Attached is a reference document for router performance.

grobinson23 Wed, 11/04/2009 - 13:31

I've had the pdf, but it doesn't go into CPU utilization. Your information is very helpful.

Thanks!

Joseph W. Doherty Wed, 11/04/2009 - 13:40

Well, assuming nothing else changed, since a 2851 is 2x the performance of your 2811, if you had it, you would expect your current CPU load to half. Further assuming about 60% is an acceptable load, you should also be able to support 2x your current load on a 2851.

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