Implementing Traffic Policers On Ex 9200 Switches; Figure 3: Token Bucket Algorithm; Figure 4: Traffic Behavior Using Policer And Burst Size - Juniper EX9200 Features Manual

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Figure 3 on page 27 represents how a policer is implemented using the token bucket
algorithm. The token allocator allocates tokens to the policer based on the configured
bandwidth limit, which is the token size multiplied by the token arrival rate.
token size x token arrival rate = policer rate (configured bandwidth limit)

Figure 3: Token Bucket Algorithm

Incoming
Packet
When a packet arrives at an interface configured with a policer, tokens that represent
the number of bits that correspond to the length of the packet are used (or "cashed in")
from the token bucket. If the token arrival rate is higher than the rate of traffic so that
there are tokens not being used, the token bucket is filled to capacity, and arriving tokens
"overflow" the bucket and are lost. The token bucket depth represents the single
user-configured burst size for the policer.
If there are tokens in the token bucket and the incoming traffic rate is higher than the
token rate (the configured policer rate, bandwidth limit), the traffic can use the tokens
until the bucket is empty. The token consumption rate can be as high as the incoming
traffic rate, which creates the burst of traffic shown in
By using the token bucket algorithm, the average bandwidth rate being allowed is close
to the configured bandwidth limit while simultaneously supporting bursty traffic, as
shown in Figure 4 on page

Figure 4: Traffic Behavior Using Policer and Burst Size

Traffic
Volume
Unused
Bursts use
Tokens
unused tokens
Chapter 5: Implementing Traffic Policers on EX 9200 Switches
Token Allocator
Token Bucket
Outgoing
Packet
27.
Unused Unused
Tokens Tokens
Figure 4 on page 27.
Bandwidth Limit
Time
27