QoS Support on the Catalyst 4000 IOS Family of Switches 221
Conforming rate <= (1540000 bits/sec * 1 byte/8 bits * 1 sec) + 8000 bytes +
1 packet = 200500 bytes
In bits per second, the conforming rate is 2.006 Mbps, assuming an average 100-byte packet size.
Because of the nature of applications that use the TCP/IP and UDP/IP protocols, use careful
planning when configuring the burst size.
Policing Accuracy
Moreover, the architecture of the Catalyst 4000 IOS Family of switches bestows policing
at a finite set of rates distributed between 32 kbps and 32 Gbps in 1-bps increments.
Because of the hardware architecture, specified rates adjust up or down to the nearest
hardware-capable rate. The adjusted policy rate is always within 1.5 percent of the
configured rate. Subsequently, two distinct policy rates must differ by at least 3 percent.
DSCP-Policed Action
The Catalyst 4000 IOS Family of switches uses the DSCP-policed concept to mark down
packets. For DSCP-policed traffic, the switch marks the frame with a DSCP value derived
from the QoS DSCP-policed mapping table. The default mapping table maps 1:1 with the
internal DSCP values; therefore, the default mark down action results in no change to the
DSCP value. The QoS DSCP-policed mapping table requires a nondefault configuration to
actually mark down packets that exceed the rate specified in the policing action.
Use the following command to configure the QoS DSCP-policed mapping tables:
qos map dscp policed
dscp-list
to
dscp mark-down-dscp
dscp-list represents up to eight DSCP values that configure to represent the DSCP mark-
down-DSCP value. Example 7-17 displays the default QoS DSCP-policed mapping table,
configures the DSCP-policed mapping table for marking down DSCP values 50 to 59 to 0,
and verifies the configuration.
Example 7-17 Displaying, Configuring, and Verifying the QoS DSCP-Policed Mapping Table
Switch#show qos map dscp policed
Policed DSCP Mapping Table (DSCP = d1d2)
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 : 30 31 32 33 34 35 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
Switch(config)#qos map dscp policed 50 51 52 53 54 55 56 57 to dscp 0
Switch(config)#qos map dscp policed 58 59 to dscp 0
Switch#show qos map dscp policed
Policed DSCP Mapping Table (DSCP = d1d2)
continues