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Chapter 10 Quality of Service (QoS)
VMG5313-B10A/-B30A Series User’s Guide
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• If there are no tokens in the bucket, the VMG stops transmitting until enough tokens are
generated.
• If not enough tokens are available, the VMG treats the packet in either one of the following ways:
In traffic shaping:
• Holds it in the queue until enough tokens are available in the bucket.
In traffic policing:
•Drops it.
• Transmits it but adds a DSCP mark. The VMG may drop these marked packets if the network is
overloaded.
Configure the bucket size to be equal to or less than the amount of the bandwidth that the interface
can support. It does not help if you set it to a bucket size over the interface’s capability. The smaller
the bucket size, the lower the data transmission rate and that may cause outgoing packets to be
dropped. A larger transmission rate requires a big bucket size. For example, use a bucket size of 10
kbytes to get the transmission rate up to 10 Mbps.
Single Rate Three Color Marker
The Single Rate Three Color Marker (srTCM, defined in RFC 2697) is a type of traffic policing that
identifies packets by comparing them to one user-defined rate, the Committed Information Rate
(CIR), and two burst sizes: the Committed Burst Size (CBS) and Excess Burst Size (EBS).
The srTCM evaluates incoming packets and marks them with one of three colors which refer to
packet loss priority levels. High packet loss priority level is referred to as red, medium is referred to
as yellow and low is referred to as green.
The srTCM is based on the token bucket filter and has two token buckets (CBS and EBS). Tokens
are generated and added into the bucket at a constant rate, called Committed Information Rate
(CIR). When the first bucket (CBS) is full, new tokens overflow into the second bucket (EBS).
All packets are evaluated against the CBS. If a packet does not exceed the CBS it is marked green.
Otherwise it is evaluated against the EBS. If it is below the EBS then it is marked yellow. If it
exceeds the EBS then it is marked red.
The following shows how tokens work with incoming packets in srTCM:
• A packet arrives. The packet is marked green and can be transmitted if the number of tokens in
the CBS bucket is equal to or greater than the size of the packet (in bytes).
• After a packet is transmitted, a number of tokens corresponding to the packet size is removed
from the CBS bucket.
• If there are not enough tokens in the CBS bucket, the VMG checks the EBS bucket. The packet is
marked yellow if there are sufficient tokens in the EBS bucket. Otherwise, the packet is marked
red. No tokens are removed if the packet is dropped.
Two Rate Three Color Marker
The Two Rate Three Color Marker (trTCM, defined in RFC 2698) is a type of traffic policing that
identifies packets by comparing them to two user-defined rates: the Committed Information Rate
(CIR) and the Peak Information Rate (PIR). The CIR specifies the average rate at which packets are
admitted to the network. The PIR is greater than or equal to the CIR. CIR and PIR values are based