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New Member

How to use rate-limit.

I'm having trouble understanding how rate-limit works. Let say I want 10Mbps and want to burst to 20Mbps how should I setup rate-limit? What I would do is the following:

rate-limit input 10000000 20000000 20000000 conform-action transmit exceed-action drop

rate-limit output 10000000 20000000 20000000 conform-action transmit exceed-action drop

but this seems to only allow me to utilize 10Mpbs and can't burst to 20Mbps. Help!!



Re: How to use rate-limit.

With the settings you listed above you're specifying a 10-Mbps average rate, with normal burst size of 20-MBytes/sec (equals 160-Mbps) and excess burst size of 20-MBytes/sec (equals 160-Mbps). Burst sizes are specified in bytes, not bits.

So bursts to at least 20-Mbps are easily covered by those values. If your router interfaces are Fast Ethernet, ATM OC-3c, or slower, you aren't really limiting your rates with those numbers.

What kind of Cisco device are you trying to run rate-limiting on? If you're not seeing traffic getting past 10-Mbps, either the speed of the interface or the routing CPU is holding you back. Also: how are you measuring the throughput?

According to the Cisco IOS Quality of Service Solutions Command Reference, Release 12.2, under "rate-limit":


"When policing traffic with CAR, Cisco recommends the following values for the normal and extended burst parameters:

normal burst = configured rate * (1 byte)/(8 bits) * 1.5 seconds

extended burst = 2 * normal burst

With the listed choices for parameters, extensive test results have shown CAR to achieve the configured rate. If the burst values are too low, then the achieved rate is often much lower than the configured rate."


So, to achieve a configured average rate of 10,000,000 following Cisco's recommendations, you would use

10000000 1875000 3750000

Similarly, to achieve a configured average rate of 20,000,000 you would use

20000000 3750000 7500000

Now, to reach your goal of 10-Mbps with burst capability to 20-Mbps, you could specify

10000000 2500000 2500000

where 2,500,000 bytes is 20 megabits exactly, and have an "achieved rate" that may be lower than configured rate. Or, you could mix and match the numbers, using the average rate from 10-Mbps and the burst figures from 20-Mbps average rate:

10000000 3750000 7500000

but that would probably result in traffic rates being over 10-Mbps on average.

I would try:

10000000 2500000 3750000 (for occasional bursts)


10000000 3750000 3750000 (for frequent bursts)

Let's say you have a FastEthernet1/0 interface running 100 full duplex. If you're able to generate a sustained traffic stream across the interface of over 20-Mbps and the router CPU is not holding you back, then the "show interface FastEthernet1/0 rate-limit" command should indicate that you've had some packets that exceeded limits imposed by the burst values and were dropped.

Hope this helps.

New Member

Re: How to use rate-limit.

Thanks for the information. The equipment in use is Cisco 7206 VXR NPE300 and I"m rate-limiting on a GigabitEthernet sub-interfaces. We're using MRTG to graph the utilization which plateaus at 10Mbps.


Re: How to use rate-limit.

Interesting. There's a 7206 VXR NSE-1 on one of my networks, rate-limiting on a Gig Ethernet interface too (although not sub-interfaced). It rate-limits by ACLs, matching each customer's assigned IP address range and capping their Internet throughput according to what their contract calls for.

MRTG is used to monitor the overall bandwidth of the Gig interface, to make sure we're getting the rate we're paying for from our upstream ISP (27-megabit committed out of a 100-megabit Fast Ethernet pipe).

Could it be that your MRTG graph needs to have its scaling parameters adjusted, so that you can see traffic above 10-megabit? I don't handle the MRTG system programming here, so I don't know if that even makes sense, or is possible. But I do know that one time on another ISP link that is monitored with MRTG, when the upstream ISP cut the bandwidth to 5-megabit we could definitely see the plateau. And it was obvious when they returned the speed to the previous level: we started seeing peaks and valleys again as traffic shot up past the 5-megabit limit.

New Member

Re: How to use rate-limit.

I happened to see this post here. I am having a question for the CAR. Are the Bc/Be here same as Bc/Be in traffice shaping? In traffic shapping Bc/Be is defined per interval, not per second. By reading this post, it seems CAR Bc/Be is bytes/sec? If it's per second, for how long the CAR allow the burst traffic? What about I am sending a 100Mbytes traffic. Will it allow 5 seconds burst? How do can control this in the CAR, I mean the burst length?

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