© 2004 Cisco Systems, Inc. All rights reserved. Printed in USA.
Presentation_ID.scr
115115115
© 2004 Cisco Systems, Inc. All rights reserved.
RST-3508
9805_05_2004_c2
Policing: How to Set the Burst Size?
• Too small: And policer drops due to burstiness inherent in all
networks
• Too large: And the entire transfer fits in the burst (especially for TCP)
• Small burst size [n*max pkt size] ok for video, voice
• Larger burst needed for TCP: 2 x [RTT * rate], good starting point
• Must evaluate how UDP traffic will react to a packet drop
• Right answer depends on the network
• Starting with IOS release 12.1.19EW1 and higher, the policer calculations can
include the 14 byte Ethernet header field and 4 byte FCS field when policing
packets; this would be enabled using the global command: qos account
layer2 encapsulation length 18
• Releases prior to this do not include these fields; the policing rate and
burst parameters configured needed to deduct the layer 2 encapsulation
length when determining the policing rate and burst, otherwise
“underpolicing” would result, particularly for smaller packet sizes in the 64
byte to 256 byte range
116116116
© 2004 Cisco Systems, Inc. All rights reserved.
RST-3508
9805_05_2004_c2
Policed DSCP Mapping
Cat4500(config)# qos map dscp policed 24 to dscp 16
Sup4#sh qos maps 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 16 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
When the Rate Is Exceeded DSCP of 24 Is Marked Down to 16
Check if the Policed DSCP Table is
Correctly Programmed
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