Mapping 295
Figure 8-7 When to Change the Default Map Settings
Because the sources for the internal DSCP values have been determined, the discussion can
now turn to how the internal DSCP values are mapped back into a frame upon egress.
Catalyst 6500 trusts inbound IP
precedence and CoS on appropriate
ports. Inbound values are subsequently
mapped to an internal DSCP value.
IP Precedence and CoS 3 mapped to DSCP 26
IP Precedence and CoS 5 mapped to DSCP 46
DSCP
Router
Trust CoS
802.1q
Trunk
Catalyst
6500
Layer 2
Switch
Switch does not
set Layer 3 Prec/
DSCP values.
•• Voice control traffic
marked with CoS 3
Voice Bearer Frames
from IP phones marked
with CoS 5
Trust IP
Precedence
IP
Softphone
•• Voice control traffic
marked with IP
Precedence 3
Voice Bearer frames
from Softphone marked
with IP Precedence 5
IP
Softphone
Corporate WAN
296 Chapter 8: QoS Support on the Catalyst 6500
DSCP-to-CoS Mapping
When the frame header is passed from the switching engine to the egress port or interface,
the internal DSCP value is mapped back to a CoS value. The internal DSCP is also written
to the ToS field of the Layer 3 IP header. The CoS value allows the frame to utilize the
egress queue scheduling as it exits the switch. Output scheduling is covered in a later
section. In addition to scheduling, if the outbound port is an ISL or 802.1q trunk, the CoS
value is written into the trunk header and transmitted to the network. The CoS value is
reflected in the user priority field. To modify and verify the configuration of the DSCP-to-
CoS mapping table, use the following commands:
(Hybrid)
set qos dscp-cos-map {
DSCP1
[
DSCP2…DSCP64
]}:{
CoS
}
ss
ss
hh
hh
oo
oo
ww
ww
qq
qq
oo
oo
ss
ss
mm
mm
aa
aa
pp
pp
ss
ss
cc
cc
oo
oo
nn
nn
ff
ff
ii
ii
gg
gg
dd
dd
ss
ss
cc
cc
pp
pp
--
--
cc
cc
oo
oo
ss
ss
--
--
mm
mm
aa
aa
pp
pp
(Native)
mls qos map dscp-cos {
DSCP1
[
DSCP2 DSCP3 DSCP4 DSCP5 DSCP6 DSCP7 DSCP8
]} to {
CoS
}
show mls qos map [ | begin Dscp-cos]
In Hybrid, a range of DSCP values can be mapped to a single CoS. If the intent is to map
DSCP 24 through 35 to CoS 3, for example, the command can be configured as follows: set
qos dscp-cos-map 24-35:3. Native mode does not permit the administrator to enter a range
of values. A maximum of eight independent DSCP values can be specified per command.
In addition, when entering individual DSCP values in Hybrid, you can enter multiple
values; however, you must separate the values with commas. Native mode requires the
DSCP values to be separated with spaces. The following two examples demonstrate how to
configure the DSCP-to-CoS mapping table. They also demonstrate the differences in
command syntax. Note that to demonstrate dependency, the following examples were
configured to reflect the changes made to the CoS-to-DSCP mapping table in previous
examples.
Example 8-27 Modifying the DSCP-to-CoS Mapping Table in Hybrid Mode
hybrid (enable) set qos dscp-cos-map 56-63:0
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 48-55:1
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 40-47:2
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 32-39:3
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 24-31:4
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 16-23:5
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 8-15:6
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 0,2,4,6:7
QoS dscp-cos-map set successfully.
hybrid (enable) set qos dscp-cos-map 1,3,5,7:7
QoS dscp-cos-map set successfully.
hybrid (enable) show qos maps config dscp-cos-map
DSCP - CoS map:
DSCP CoS
Mapping 297
Policed DSCP Mark-Down Mapping
The Catalyst 6500 has an additional feature that allows it to mark down DSCP values, based
on internal policed DSCP mark-down tables. Contingent on configured policing param-
eters, instead of dropping out of profile frames, the mark-down table enables the adminis-
trator to define DSCP translations. This maps the previous internal DSCP value to another
defined “marked-down” DSCP value for frames violating the configured policing contract.
The PFC hardware version, either a PFC1 or PFC2, determines how many mark-down
tables are available.
The Catalyst 6500 with a PFC1 only provides support for the single-rate policing function.
As a result, the option of configuring the policed DSCP mark-down table for frames
violating the normal traffic rate is available with the PFC1. On the other hand, the PFC2
provides an additional enhancement beyond what is available with the PFC1. The PFC2 can
support a dual-rate policer. With the PFC2, two mark-down tables can be altered, a table for
-------------------------------- ---
56-63 0
48-55 1
40-47 2
32-39 3
24-31 4
16-23 5
8-15 6
0-7 7
Example 8-28 Modifying the DSCP-to-CoS Mapping Table in Native Mode
native(config)# mls qos map dscp-cos 56 57 58 59 60 61 62 63 to 0
native(config)# mls qos map dscp-cos 48 49 50 51 52 53 54 55 to 1
native(config)# mls qos map dscp-cos 40 41 42 43 44 45 46 47 to 2
native(config)# mls qos map dscp-cos 32 33 34 35 36 37 38 39 to 3
native(config)# mls qos map dscp-cos 24 25 26 27 28 29 30 31 to 4
native(config)# mls qos map dscp-cos 16 17 18 19 20 21 22 23 to 5
native(config)# mls qos map dscp-cos 8 9 10 11 12 13 14 15 to 6
native(config)# mls qos map dscp-cos 0 1 2 3 4 5 6 7 to 7
native# show mls qos map | begin Dscp-cos
Dscp-cos map: (dscp= d1d2)
d1 : d2 0 1 2 3 4 5 6 7 8 9
-------------------------------------
0 : 07 07 07 07 07 07 07 07 06 06
1 : 06 06 06 06 06 06 05 05 05 05
2 : 05 05 05 05 04 04 04 04 04 04
3 : 04 04 03 03 03 03 03 03 03 03
4 : 02 02 02 02 02 02 02 02 01 01
5 : 01 01 01 01 01 01 00 00 00 00
6 : 00 00 00 00
(text omitted)
Example 8-27 Modifying the DSCP-to-CoS Mapping Table in Hybrid Mode (Continued)
298 Chapter 8: QoS Support on the Catalyst 6500
traffic violating the normal rate and streams violating the excess rate. Table 8-13 lists the
default mapping values for both normal and excess rate policed DSCP mark-down tables.
To alter how internal DSCP values are mapped to policed DSCP mark-down values, use the
following commands:
(Hybrid)
set qos policed-dscp-map [excess-rate | normal-rate] {
DSCP1
[
DSCP2…DSCP64
]}:{
DSCP
}
show qos maps config policed-dscp-map [excess-rate | normal-rate]
(Native)
mls qos map policed-dscp {max-burst | normal-burst} {
DSCP1
[
DSCP2 DSCP3 DSCP4 DSCP5
DSCP6 DSCP7
DSCP8
]} to {
DSCP
}
show mls qos map [ | begin Policed-dscp]
With the PFC2, the additional excess-rate and normal-rate keywords are available in
Hybrid mode, and the max-burst and normal-burst keywords are available in Native
mode. If neither of the keywords are specified, the configured DSCP mappings apply to the
normal rate table. With the PFC1, the preceding keywords are not available.
When configuring the excess rate table on the PFC2, if changes have been made to the
normal rate table, those alterations are independent from policies created for the excess rate
Table 8-13 Normal And Excess Rate Default Policed DSCP Mark-Down Values
Internal
DSCP
Mark-
Down
DSCP
Internal
DSCP
Mark-
Down
DSCP
Internal
DSCP
Mark-
Down
DSCP
Internal
DSCP
Mark-
Down
DSCP
0 0 16 16 32 32 48 48
1 1 17 17 33 33 49 49
2 2 18 18 34 34 50 50
3 3 19 19 35 35 51 51
4 4 20 20 36 36 52 52
5 5 21 21 37 37 53 53
6 6 22 22 38 38 54 54
7 7 23 23 39 39 55 55
8 8 24 24 40 40 56 56
9 9 25 25 41 41 57 57
10 10 26 26 42 42 58 58
11 11 27 27 43 43 59 59
12 12 28 28 44 44 60 60
13 13 29 29 45 45 61 61
14 14 30 30 46 46 62 62
15 15 31 31 47 47 63 63
Mapping 299
table. This is attributed to the mark-down operation occurring after the frame has been
processed through the policing function. Consider the scenario where a dual-rate policer is
configured, and traffic streams violating the configured contract rates have their DSCP
values marked down in both instances. If a frame violates only the normal traffic rate
profile, it is marked down according to how the original internal DSCP value is mapped to
other DSCP values in the normal rate table. When the same frame reaches the excess rate
policer, it still possesses the original internal DSCP value. Therefore, if the frame exceeds
the excess rate, the mark-down value is determined based on how the original DSCP value
is mapped in the excess rate table. The following examples demonstrate how to modify and
verify the policed DSCP tables in Hybrid and Native modes. Example 8-29 shows how to
map DSCP 24 to DSCP 8 in the mark-down table in Hybrid, for a switch with a PFC1.
Example 8-30 shows how to map DSCP 24 to DSCP 8 for the normal rate policer and DSCP 24
to DSCP 0 for the excess rate policer in Native mode.
Example 8-29 Modifying the Policed DSCP Mark-Down Table in Hybrid Mode
hybrid (enable) set qos policed-dscp-map 24:8
QoS policed-dscp-map set successfully.
hybrid (enable) show qos map config policed-dscp-map
DSCP - Policed DSCP map:
DSCP Policed DSCP
-------------------------------- ------------
0 0
1 1
2 2
3 3
4 4
5 5
6 6
7 7
8,24 8
(text omitted)
Example 8-30 Modifying the Policed DSCP Mark-Down Table in Native Mode
native(config)# mls qos map policed-dscp normal-burst 24 to 8
native(config)# mls qos map policed-dscp max-burst 24 to 0
native# show mls qos map | begin Policed-dscp
Normal Burst Policed-dscp map: (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 08 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
Maximum Burst Policed-dscp map: (dscp= d1d2)
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