03-25-2011 11:06 AM - edited 03-04-2019 11:52 AM
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.
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04-08-2011 11:33 AM
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
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