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 Burst size—The capacity of the token bucket (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.
How line rate works
With line rate configured on an interface, all packets to be sent out the interface are firstly handled by the
token bucket of line rate. If enough tokens are available 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.
Figure 395 Line rate implementation
With a token bucket used for traffic control, when tokens are available in the token bucket, the bursty
packets can be transmitted; if no tokens are available, packets cannot be transmitted until new tokens are
generated in the token bucket. In this way, the traffic rate is restricted to the rate for generating tokens,
limiting traffic rate and allowing bursty traffic.
Priority mapping
What is priority mapping
When a packet enters a network, it is marked with a certain priority to indicate its scheduling weight or
forwarding priority. Then, the intermediate nodes in the network process the packet according to the
priority.
When a packet enters a device, the device assigns to the packet a set of predefined parameters
(including the 802.1p precedence, DSCP values, IP precedence, and local precedence).
ï‚· For more information about 802.1p precedence, DSCP values, and IP precedence, see Packet
precedences.
ï‚· Local precedence is a locally significant precedence that the device assigns to a packet. A local
precedence value corresponds to an output queue. Packets with the highest local precedence are
processed preferentially.
The device provides the following priority trust modes on a port:
 Trust packet priority—The device assigns to the packet the priority parameters corresponding to the
packet’s priority from the mapping table.
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