QoS--->LLQ Problem!

blackhat2020 · ‎10-04-2008

Hi,im about configuring llq for my voice traffic on my wan routers but for some reasone im confused.this is my sample config:

policy-map test

class VOICE

priority 17 X

as you can see in above configuration i can guaranty 17Kbps bandwith for my voice traffic but after that number 17 you can enter a number(X) witch is burst size in byte.so what does it mean? i mean if i enter 2125(byte) in burst section my QoS lets my voice traffic to have 17kbps at minimum and can extend this bandwith to 17kbps+(2125byte=17kb)=34kbps. is this correct??please help me.thanks

Giuseppe Larosa · ‎10-04-2008

Hello Sevan,

you can configure a burst size but its effect is not that of doubling the allowed bandwidth: it allows to go over the rate to accomodate a short peak of traffic (a burst of packets ) up to the specified burst size.

Then the steady allowed rate is the rate you have configured.

in the command reference

Optional) Specifies the burst size in bytes. The burst size configures the network to accommodate temporary bursts of traffic. The default burst value, which is computed as 200 milliseconds of traffic at the configured bandwidth rate, is used when the burst argument is not specified. The range of the burst is from 32 to 2000000 bytes.

http://www.cisco.com/en/US/docs/ios/qos/command/reference/qos_n1.html#wp1014542

You can think of the burst as additional credits you can use from time to time.

In the case of VOIP the used bandwidth depends on the used codec (G.711 or G.729 for example) and there is very little variation: VOIP traffic is far more predictable then data traffic.

Hope to help

Giuseppe