174 Chapter 6: QoS Features Available on the Catalyst 2950 and 3550 Family of Switches
dscp-list represents up to eight DSCP values. The switch marks down frames with these
DSCPS to the DSCP value specified by the mark-down-dscp value when exceeding the
configured rate. To configure the exceed-action of the traffic-rate policer, use the following
command:
police
rate-bps burst-byte
[exceed-action {drop | policed-dscp-transmit}]
Example 6-24 illustrates a user configuring and verifying the policed DSCP mapping table.
Burst Size
Because of the behavior of TCP/IP and UDP applications, packet drops due to policing may
significantly impact traffic throughput and may result in a packet-per-second throughput far
below the configured policer rate. The burst parameter of policing attempts to handle this
behavior by allowing periodic surges of traffic into the bucket.
Configuration of the burst size follows several other Catalyst platform recommendations.
For TCP applications, use the following formula to calculate the burst size parameter used
for policing:
<Burst> = 2 * <RTT> * <Rate>
RTT defines the approximate round-trip time for a TCP session. If RTT is unknown, use a
RTT value of 1 ms to 1 second depending on estimated latency. The burst calculation for a
rate of 64 kbps and an RTT of 100 ms is as follows
<Burst> = 2 * <.100 sec> * <64000 bits/sec>
<Burst> = 12800 bits = 1600 bytes
Nevertheless, from an application standpoint, rate policing always results in actual rates
less than the configured rate regardless of the burst size. UDP applications react closer to
the configured rate in bits per second; nevertheless, some UDP applications retransmit
heavily upon packet loss resulting in performance far less than the configured rate. In brief,
carefully consider burst rate and its effects on application before applying policers.
Example 6-24 User Configuring and Verifying the Policed DSCP Mapping Table
Switch#configure terminal
Switch(config)#mls qos map policed-dscp 30 31 32 33 34 35 to 20
Switch(config)#end
Switch#show mls qos map policed-dscp
Policed-dscp map:
d1 : d2 0 1 2 3 4 5 6 7 8 9
---------------------------------------
0 : 00 01 02 03 04 05 06 07 08 09
1 : 10 11 12 13 14 15 16 17 18 19
2 : 20 21 22 23 24 25 26 27 28 29
3 : 20 20 20 20 20 20 36 37 38 39
4 : 40 41 42 43 44 45 46 47 48 49
5 : 50 51 52 53 54 55 56 57 58 59
6 : 60 61 62 63