H3C Technologies H3C WX6000 Series Access Controllers - Manual (Page 533)

46-4
Line Rate
Line rate is a traffic control method using token buckets. The line rate of a physical interface specifies
the maximum rate for forwarding packets (including critical packets). Line rate can limit all the packets
passing a physical interface.
Traffic evaluation and token bucket
A token bucket can be considered as a container holding a certain number of tokens. The system puts
tokens into the bucket at a set rate. When the token bucket is full, the extra tokens will overflow.
Figure 46-2 Evaluate traffic with the token bucket
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