QoS for VoiP

Unanswered Question
May 28th, 2007

From this doc:


Burst?defines the maximum number of tokens that the bucket can hold at any one time. To sustain the specified traffic rate, burst should be no less than the rate times the interval. Another consideration is that the maximum-size packet must fit into the bucket.

To determine the burst parameter, use this equation:

Burst = (Rate [bps]) * 0.00025 [sec/interval]) or (maximum packet size [bits]), whichever is greater.

For example, if you want to calculate the minimum burst value needed to sustain a rate of 1 Mbps on an Ethernet network, the rate is defined as 1 Mbps and the maximum Ethernet packet size is 1518 bytes. The equation is:

Burst = (1,000,000 bps * 0.00025) or (1518 bytes * 8 bits/byte) = 250 or 12144.

The larger result is 12144, which you round to 13 kbps.

Note: In Cisco IOS? Software, the policing rate is defined in bits per second (bps), as opposed to kbps in Catalyst OS (CatOS). Also in Cisco IOS Software, the burst rate is defined in bytes, as opposed to kilobits in CatOS

From my CAT6500

CAT6500> (enable) set qos policer aggregate COMMUNICATOR-VOICE rate 128 burst ?

<burst> Burst 1..32000 (kb)

From SRND and QoS enterprise guide:


set qos policer aggregate COMMUNICATOR-VOICE rate 128 burst 8000 drop

Why the burst is specfied to 8000 ?

How can i calculate it?


I have this problem too.
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Anthony Mile Tue, 05/29/2007 - 00:28

hi guys, am configuring my router to support Linksys wireless skype version 3.2 and it doesnt work. can you advice me on what might be the problem with this

guruprasadr Tue, 05/29/2007 - 01:45

HI Justin, [Pls Rate if Helps]

Sample QOS Configuration for Voice & Data:

class-map match-any VoIP

match access-group 100 [subnet of VoIP]

class-map match-any RTP

match ip precedence 5

match ip precedence 4



policy-map low_level_QoS_backbone [as VoIP]

class RTP

priority 2000

class class-default


policy-map top_level_QoS_backbone_8M[as Data]

class class-default

shape average

service-policy low_level_QoS_backbone

policy-map top_level_QoS_backbone_

class class-default

shape average

service-policy low_level_QoS_backbone

policy-map Marking

class VoIP

set ip precedence 5

Pls Rate if Helps ! !

Best Regards,

Guru Prasad R


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