rate-limit burst-normal burst-max calculation

Unanswered Question
Apr 9th, 2010

Hi All,

We have one router configured with rate-limit on LAN port to restrict incomming traffic to 16MB. The rate limit command is configured since long and now we want to rate-limit it to 17MB to accomodate 1MB more. I am confused about how to calculate "burst-normal, burst-max". Can anybody please advise.

Command Syntax:

rate-limit {input | output} [dscp dscp-value] [access-group [rate-limit] acl-index] bps burst-normal burst-max conform-action conform-action exceed-action exceed-action

Current config on Router:

interface GigabitEthernet0/1
rate-limit input access-group 126 16000000 6325000 12650000 conform-action transmit exceed-action drop

I have this problem too.
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Giuseppe Larosa Fri, 04/09/2010 - 14:38

Hello Manoj,

the normal burst size and the max burst size are the size of buffers expressed in bytes that you can allocate when using the CAR feature.

The CAR design rules suggests that burst size should be big enough to accomodate 1,5 sec of traffic at the desired speed.

The other parameter burst-max provides an extra buffer that can be used to provide some more space for traffic at startup.

What counts is the ratio extended burst size / normal burst size and the first one has to be bigger then the normal burst size.

So for changing from 16 Mbps (bit per seconds) to 17 Mbps you could even keep the current values with no noticeable effects.

I can also say that current parameters are even bigger then recommended ones

Here it is a link to 12.4 QoS command reference that explains all the parameters of

rate-limit command

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_q1.html#wp1054922

see the usage guidelines

Hope to  help

Giuseppe

Lei Tian Sat, 04/10/2010 - 18:52

Hi Manoj,

Rate limit uses token bucket algorithm. The BC or normal burst is the depth of the bucket. Packet will be transmitted if there is enough token is the bucket. The max burst is used when there is not enough token in bucket; 'max_burst - normal_burst' number of bytes can be borrowed.

Rate limit is the legacy QoS method; you can use MQC and policing to achieve your goal.

HTH,

Lei Tian

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