3Com Baseline 2928 PWR Plus User Manual page 469

Figure 2-8 Evaluate traffic with the token bucket
Packets to be sent
through this interface
Packet
classification
The evaluation for the traffic specification is based on whether the number of tokens in the bucket can
meet the need of packet forwarding. If the number of tokens in the bucket is enough to forward the
packets (generally, one token is associated with a 1-bit forwarding authority), the traffic conforms to the
specification, and the traffic is called conforming traffic; otherwise, the traffic does not conform to the
specification, and the traffic is called excess traffic.
A token bucket has the following configurable parameters:
Mean rate: At which tokens are put into the bucket, namely, the permitted average rate of traffic. It
is usually set to the committed information rate (CIR).
Burst size: the capacity of the token bucket, namely, the maximum traffic size that is permitted in
each burst. It is usually set to the committed burst size (CBS). The set burst size must be greater
than the maximum packet size.
One evaluation is performed on each arriving packet. In each evaluation, if the number of tokens in the
bucket is enough, the traffic conforms to the specification and the corresponding tokens for forwarding
the packet are taken away; if the number of tokens in the bucket is not enough, it means that too many
tokens have been used and the traffic is excessive.
The working mechanism of line rate
With line rate configured on an interface, all packets to be sent through the interface are firstly handled
by the token bucket of line rate. If there are enough tokens in the token bucket, packets can be
forwarded; otherwise, packets are put into QoS queues for congestion management. In this way, the
traffic passing the physical interface is controlled.
Tokens are put into the
bucket at the set rate
Packets sent
Token
bucket
Packets dropped
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