Received
DSCP
Policy DSCP
802.1p
Priority
Policy Name
(Optional)
001100
17
6
Level 6
001101
16
4
Level 4
1. Determine whether the DSCPs already have priority assignments, which could indicate use by existing
applications. This is not a problem as long as the configured priorities are acceptable for all applications
using the same DSCP. (See Note on changing a priority setting on page 202. Also, a DSCP must have a
priority configured before you can assign any QoS classifiers to use it. See Differentiated Services
Codepoint (DSCP) mapping on page 200.)
2. After configuring the DSCP policies for the codepoints you want to use, assign the policies to the
codepoints in the selected packet type.
An example of policy assignment to outbound packets on the basis of the DSCP in the packets received
from upstream devices is shown below. The specified DSCP policies overwrite the original DSCPs on the
selected packets, and use the 802.1p priorities previously configured.
switch(config)# qos type-of-service diff-services 001100 dscp 17
switch(config)# qos type-of-service diff-services 001101 dscp 16
switch(config)# show qos type-of-service
Type of Service : Differentiated Services
Codepoint DSCP Policy | Priority
--------- ----------- + -----------
000000 | No-override
000001 | No-override
000010 | No-override
000011 | No-override
000100 | No-override
000101 | No-override
000110 | No-override
000111 | No-override
001000 001011 | 7
001001 | No-override
001010 | 1
001011 | 7
001100 010001 | 6
001101 010000 | 4
Details of QoS IP ToS
IP packets include a Type of Service (ToS) byte. The ToS byte includes:
Precedence Bits: This element is a subset of the DSCP and is composed of the upper 3 bits of the ToS byte.
When configured to do so, the switch uses the precedence bits to determine a priority for handling the
Chapter 8 Quality of Service (QoS): Managing bandwidth
effectively
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