ASK THE EXPERTS - QoS with Cisco expert Sarala Akella

[ciscomoderator]

Welcome  to the Cisco Support Community Ask the Expert conversation. This is an  opportunity to learn about configuration and troubleshooting Quality of  Service on routers with Cisco expert Sarala Akella. Sarala is  a customer support engineer at the Cisco Technical Assistance Center.  She currently works in the WAN team where she focuses on various WAN  related issues along with QoS issues on various interfaces. Sarala has  been with Cisco for 11 years and has worked as a software engineer in  the Network Software and Systems Technology Group. She holds a masters  degree in computer engineering from Santa Clara University and a masters  degree in mathematics from Osmania University, India.

Remember to use the rating system to let Sarala know if you have received an adequate response.

Sarala  might not be able to answer each question due to the volume expected  during this event. Remember that you can continue the conversation on  the WAN, Routing and Switching discussion forum shortly after the event.  This event lasts through April 8, 2011. Visit this forum often to view responses to your questions and the questions of other community members.

[sakella]

The default burst size is calculated for 1/8th of the policed rate of traffic.
For example the guaranteed bandwidth of 40,000 bps, it is equal to 8,000 bits or 1,000 bytes.

It is  always a good idea to leave the burst size to be calculated by the IOS,

which will use the inbuilt algorithm which is approximately 1/8 of the policed rate.

also please note there little variation due  card specific burst size changes.

The leaky bucket that Traffic Shaping uses has a maximum capacity of Bc plus Be. Hence, the

maximum burst allowed is equal to Bc + Be. A sum of tokens equivalent to Bc is added to

the token bucket in every Tc time interval. When the router needs to transmit a packet on

an exit interface with Traffic Shaping enabled, the packet is transmitted only if there

are sufficient tokens in the token bucket. Otherwise, the packet is buffered in the queues

set up on the PVC until sufficient tokens are available. In this way, Traffic Shaping

ensures that the long-term transmission rate does not exceed the established rate at which

tokens are replaced in the bucket and allows the network to manage congestion.

Configure Be only if the CIR value is less than the AR.  For voice, Be = 0.

** The token bucket is filled with Bc tokens in each Tc

The leaky bucket that Traffic Shaping uses has a maximum capacity of Bc plus Be. Hence,

the maximum burst allowed is equal to Bc + Be. A sum of tokens equivalent to Bc is added

to the token bucket in every Tc time interval. When the router needs to transmit a packet

on an exit interface with Traffic Shaping enabled, the packet is transmitted only if there

are sufficient tokens in the token bucket. Otherwise, the packet is buffered in the queues

set up on the PVC until sufficient tokens are available. In this way, Traffic Shaping

ensures that the long-term transmission rate does not exceed the established rate at which

tokens are replaced in the bucket and allows the network to manage congestion.

A token bucket is a formal definition of a rate of transfer. It has three components: a

burst size, a mean rate, and a time interval (Tc).

mean rate = burst size / time interval

CIR = Bc / Tc