Specifications

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A constant stream of 64 byte packets arrives at the interface.
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PFC3B is used, so L2 Header plus Data is counted.
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Burst set to 5,000.
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Conforming traffic will be transmitted as per configuration statement.
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Traffic exceeding the rate will be dropped as per configuration statement.
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Token Rate Replenishment for each interval is calculated as 10Mb / 4000 = 10,000,000 / 4000 = 2500
The following table provides an insight into how this policer works.
Table 19. Policer in Action
Time Interval Bits clocked in
interval
Tokens at
start of
interval
How many packets
can be sent
Number of bits
that are forwarded
Number of bits
not forwarded
Tokens left over
at end of interval
T0 25000 40,000 48 24,576 424 15,424
T1 25000 17,924 35 17,920 7080 4
T2 25000 2504 4 2048 22952 456
T3 25000 2946 5 2560 22440 386
T4 25000 2836 5 2560 22440 276
T5 25000 2776 5 2560 22440 216
T6 25000 2716 5 2560 22440 156
T7 25000 2656 5 2560 22440 96
T8 25000 2596 5 2560 22440 36
T9 25000 2536 4 2048 22952 488
T10 25000 2988 5 2560 22440 428
As the table shows, at time interval T0, the bucket has a full complement of tokens. The initial stream of packets
arrives and 48 packets can be sent. The 48 packets equates to 24,576 bits (48 packets x 512 bits per packet), so
24,576 tokens are removed from the bucket leaving 15,424 from the original 40,000 tokens left for the next interval.
At time interval T1, another 2500 tokens are added to the bucket. The number of packets forwarded this time is
reduced from the previous interval, as there are fewer tokens in the bucket. One interesting point is in time interval
T9 where the forwarded packet count drops to 4 packets, however, this recovers to 5 packets in the next interval. A
pattern would then become established with roughly 5 packets being sent every 7 or 8 intervals and 4 packets being
sent every 8th or 9th interval.
Statistically over a period of time, the number of packets sent should closely equate to the defined Rate. If it is
assumed that for every 9 intervals we send 8 lots of 5 packets and one lot of 4 packets (using a rough average from
the table above), this would equate to a final Rate of just over 10Mb every second ([4000 / 9] x [[8 x 5 x 512] + [1 x 4
x 512]] = 10,002,242 bits =~ 10Mbps).
8.9.1. DSCP Markdown Maps
DSCP Markdown maps are used when the policer is defined to markdown out of profile traffic instead of dropping it.
Out of profile traffic is defined as that traffic that exceeds the defined burst setting.
A default DSCP markdown map is set up when QoS is enabled. This default markdown map is listed in Table 3
earlier in the document. The CLI allows an administrator to modify the default markdown map using the map
policed-dscp command. An example of this is shown below
Cat6500(config)# mls qos map policed-dscp normal-burst 32 to 16
This example defines a modification to the default policed DSCP from 32 to 16