290 Chapter 8: QoS Support on the Catalyst 6500
Example 8-22 Configuring and Applying a QoS Policy in Native Mode (Continued)
Policy Map VideoConf
class Control-traffic
set ip dscp 26
After the policy map parameters have been established, you must apply the QoS policy to
the appropriate interface to complete the process. Here, the desired interface is Gigabit 5/
1, and the service-policy input {name} command is configured referencing policy map
VideoConf.
native(config)# interface gigabitEthernet 5/1
native(config-if)# service-policy input VideoConf
native# show policy-map interface gigabitEthernet 5/1
GigabitEthernet5/1
service-policy input: VideoConf
class-map: Control-traffic (match-any)
0 packets, 0 bytes
5 minute offered rate 0 bps, drop rate 0 bps
match: access-group 110
(text omitted)
native# show mls qos ip ingress
QoS Summary [IP]: (* - shared aggregates, Mod - switch module)
Int Mod Dir Cl-map DSCP AgId Trust FlId AgForward-Pk AgPoliced-Pk
--------------------------------------------------------------------------------
Gi5/1 1 I Control-t 26 3 No 0 0 0
(text omitted)
native# show queueing interface gigabitEthernet 5/1
Interface GigabitEthernet5/1 queueing strategy: Weighted Round-Robin
Port QoS is enabled
Port is untrusted
Default COS is 2
(text omitted)
Finally, after the appropriate steps for MQC have been completed, you can verify behavior.
As demonstrated, show policy-map interface {type num} and show mls qos ip ingress
detail the policy map class information. Specifically, the outputs include the ACL refer-
enced in the class map and the DSCP value used to mark all conforming traffic. Based on
the information in Table 8-11, and the output extracted from show queueing interface
{type num}, note that the port is configured as untrusted. This implies all traffic matching
the ACL is marked with DSCP 26. Recall that set ip dscp overrides interfaces with an
untrusted port trust state. All remaining traffic is marked with the default CoS value of 2.
As mentioned previously, this configuration is possible with Cisco IOS Release
12.1(12c)E1 and later. For earlier versions, it is necessary to configure a policer to mark all
desired traffic. Policing is covered later in the chapter.
Mapping
After a frame header has been forwarded from the ingress port to the switching engine, and
the appropriate priority value has been established, the PFC determines an internal DSCP
value for the frame. This value is used to signify how a frame is handled while it traverses
the switch, and ultimately how it is scheduled upon egress. A DSCP value is applied to all
frames processed by the switching engine, regardless of traffic type. This behavior is also
independent of the operating system. However, the manner for configuring the tables is
different between Native mode and Hybrid mode. Table 8-12 shows the default mapping
values used by the 6500.
Mapping 291
The Catalyst 6500 determines the internal DSCP value based on the trust state specified
either at the port or interface level, or based on the trust keyword or DSCP/IP precedence
value specified in an ACL. The following sections detail how the internal DSCP values are
derived from the appropriate sources and how internal DSCP values map to egress CoS.
These topics are covered in the following order:
• CoS-to-DSCP Mapping
• Precedence-to-DSCP Mapping
• DSCP-to-CoS Mapping
• Policed DSCP-to-Mark Down Mapping
CoS-to-DSCP Mapping
If the ingress port or interface has been configured to trust-cos, the internal DSCP value is
derived from the arriving CoS on the 802.1q or ISL trunk. Likewise, because an ACL or
policy map class supersedes the port trust state, if they are configured to trust the incoming
CoS value, the internal DSCP is again derived from the inbound CoS. One caveat to
consider is traffic arriving on an untrusted port. Although incoming frames are marked with
the default CoS setting configured for the port or interface, in this case the internal DSCP
value is derived from the default ACL. By default, the default ACL applies a DSCP value
of zero. As a result, if the desire is to maintain the CoS established by the default CoS value
at the interface, it is necessary to use an ACL or policy map class to trust CoS, or modify
the default DSCP value in the default ACL.
(Hybrid)
set qos cos-dscp-map {
DSCP1 DSCP2 DSCP3 DSCP4 DSCP5 DSCP6 DSCP7 DSCP8
}
ss
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--
--
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(Native)
Table 8-12 Default Mapping Values
CoS-to-DSCP
Mapping Precedence-to-DSCP Mapping DSCP-to-CoS Mapping
CoS 0 = DSCP 0 IP precedence 0 = DSCP 0 DSCP 0–7 = CoS 0
CoS 1 = DSCP 8 IP precedence 1 = DSCP 8 DSCP 8–15 = CoS 1
CoS 2 = DSCP 16 IP precedence 2 = DSCP 16 DSCP 16–23 = CoS 2
CoS 3 = DSCP 24 IP precedence 3 = DSCP 24 DSCP 24–31 = CoS 3
CoS 4 = DSCP 32 IP precedence 4 = DSCP 32 DSCP 32–39 = CoS 4
CoS 5 = DSCP 40 IP precedence 5 = DSCP 40 DSCP 40–47 = CoS 5
CoS 6 = DSCP 48 IP precedence 6 = DSCP 48 DSCP 48–55 = CoS 6
CoS 7 = DSCP 56 IP precedence 7 = DSCP 56 DSCP 56–63 = CoS 7
292 Chapter 8: QoS Support on the Catalyst 6500
mls qos map cos-dscp {
DSCP1 DSCP2 DSCP3 DSCP4 DSCP5 DSCP6 DSCP7 DSCP8
}
show mls qos map [ | begin Cos-dscp map]
Table 8-12 shows the default mapping settings for the CoS-to-DSCP table. To modify and
verify the default or modified mapping settings for the CoS-to-DSCP table, use the
preceding commands. These commands enable an administrator to map one DSCP value to
a CoS value. As a result, referencing the preceding commands, {DSCP1} maps to CoS 0
and {DSCP2} maps to CoS 1. This pattern applies to all DSCP values, concluding with
{DSCP8}, which maps to CoS 7. Examples 8-23 and 8-24 demonstrate how to modify the
default mapping values in both Hybrid and Native modes, respectively.
The values mapped in the preceding examples are not recommended settings. Instead, they
demonstrate the capability of the CoS-to-DSCP mapping feature. It is normally not
necessary to modify the CoS-to-DSCP mapping table from its defaults. If you do make
alterations, however, carefully consider how the values are mapped and propagate the
modifications across the entire network for consistency. Changes performed to this table
impact the performance of the DSCP-to-CoS table, which needs to be considered. When
configuring the mapping feature on WS-X6224/WS-X6248 and WS-X6324/WS-X6348
linecards, additional steps are required. In Hybrid, based on limitations previously
discussed, a port configured to trust CoS sustains the inbound CoS setting by applying an
ACE with the trust-cos keyword. The switch then derives the DSCP value from the CoS of
the incoming tagged frame. With Native mode, trust is not a configurable option for these
Example 8-23 Modifying the CoS-to-DSCP Mapping Table in Hybrid Mode
hybrid (enable) set qos cos-dscp-map 56 48 40 32 24 16 8 0
QoS cos-dscp-map set successfully.
hybrid (enable) show qos map config cos-dscp-map
CoS - DSCP map:
CoS DSCP
--- ----
0 56
1 48
2 40
3 32
4 24
5 16
6 8
7 0
Example 8-24 Modifying the CoS-to-DSCP Mapping Table in Native Mode
native(config)# mls qos map cos-dscp 56 48 40 32 24 16 8 0
native# show mls qos map | begin Cos-dscp map
Cos-dscp map:
cos: 0 1 2 3 4 5 6 7
------------------------------------
dscp: 56 48 40 32 24 16 8 0
(text omitted)
Mapping 293
modules at the port or interface level. All ports default to untrusted. As a result, to maintain
the local port CoS setting and use it to derive the internal DSCP, a policy map class must be
configured and applied to the applicable interface. Otherwise, the value associated with the
default ACL is utilized to obtain the internal DSCP value, which is statically defined as zero.
Precedence-to-DSCP Mapping
Similar to the CoS-to-DSCP mapping, internal DSCP values can also be derived from
arriving IP precedence values. The default IP precedence-to-DSCP mapping is shown in
Table 8-12. Frames arriving at ports or interfaces configured with the trust-ipprec or trust
ip-precedence keywords obtain their internal DSCP value from the precedence of the
ingress traffic. This same feature applies to ACLs and policy map classes. The feature is
also available for WS-X6224/WS-6248 and WS-X6324/WS-X6348 modules. Because this
configuration is not supported at the physical interface or port level for these linecards,
using either Hybrid or Native modes, however, an ACL or a policy map class must be
configured. The following commands enable the administrator to modify the default
mappings and verify the configuration:
(Hybrid)
set qos ipprec-dscp-map {
DSCP1
} {
DSCP2
} {
DSCP3
} {
DSCP4
} {
DSCP5
} {
DSCP6
} {
DSCP7
}
{
DSCP8
}
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--
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(Native)
mls qos map ip-prec-dscp {
DSCP1
} {
DSCP2
} {
DSCP3
} {
DSCP4
} {
DSCP5
} {
DSCP6
} {
DSCP7
}
{
DSCP8
}
show mls qos maps [ | begin IpPrec ]
As seen with CoS-to-DSCP mapping, {DSCP1} is associated with IP precedence 0 and
{DSCP2} corresponds with IP precedence 1. The pattern then concludes with {DSCP8},
which maps to IP precedence 7. Examples 8-25 and 8-26 demonstrate how to alter the
default IP precedence-to-DSCP mapping table and how to verify the configuration.
Example 8-25 Modifying the IP Precedence-to-DSCP Mapping Table in Hybrid Mode
hybrid (enable) set qos ipprec-dscp-map 56 48 40 32 24 16 8 0
QoS ipprec-dscp-map set successfully.
hybrid (enable) show qos maps config ipprec-dscp-map
IP-Precedence - DSCP map:
IP-Prec DSCP
------- ----
0 56
1 48
2 40
3 32
4 24
5 16
6 8
7 0
hybrid (enable)
294 Chapter 8: QoS Support on the Catalyst 6500
Example 8-26 Modifying the IP Precedence-to-DSCP Mapping Table in Native Mode
Once again, the values mapped in the preceding examples are not recommended settings.
They demonstrate the capability of the IP precedence-to-DSCP mapping feature. The
default settings are sufficient for most instances, unless specific requirements dictate
otherwise. If the default settings are modified, however, it is imperative to maintain consis-
tency across the entire network. One example where it may be necessary to modify the
default map settings is when deploying voice in the campus. For voice streams, DSCP 26
is recommended for control traffic, whereas DSCP 46 is recommended for the actual voice
packets. This is critical in environments where QoS policies are implemented using DSCP
values. To demonstrate this scenario, consider the network in Figure 8-7.
The Catalyst 6500 in Figure 8-7 is trunked to a Layer 2 switch, which is trusting the
incoming CoS set by the IP Phones. Also two laptops configured with IP Softphone are
attached directly to the Catalyst 6500. IP Softphones are capable of marking voice packets
with an IP precedence of 5. The router attached to the corporate WAN implements QoS
using DSCP-based policies. In this case, to ensure the proper end-to-end handling for the
voice traffic, it is necessary to alter the default values in the CoS-to-DSCP and IP prece-
dence-to-DSCP mapping tables. For both tables, CoS 3 and IP precedence 3 are mapped to
DSCP 26, and CoS 5 and IP precedence 5 are mapped to DSCP 46. Modifying the mapping
tables to account for the voice traffic ensures the voice streams receive the appropriate
service levels when forwarded to DSCP-capable devices.
native(config)# mm
mm
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11
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00
00
native# show mls qos maps | begin IpPrec
IpPrecedence-dscp map:
ipprec: 0 1 2 3 4 5 6 7
------------------------------------
dscp: 56 48 40 32 24 16 8 0
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