A General View of QoS on the Catalyst Platforms 59
would be necessary to use the Layer 3 marking to mark a Layer 2 CoS value on these
downstream packets.
The commands for handling these mappings differs between platforms, but the command
for setting a CoS-to-DSCP map on the Catalyst 6500 Family of switches is as follows:
ss
ss
ee
ee
tt
tt
qq
qq
oo
oo
ss
ss
cc
cc
oo
oo
ss
ss
--
--
dd
dd
ss
ss
cc
cc
pp
pp
--
--
mm
mm
aa
aa
pp
pp
{dd
dd
ss
ss
cc
cc
pp
pp
11
11
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
22
22
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
33
33
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
44
44
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
55
55
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
66
66
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
77
77
,,
,,
dd
dd
ss
ss
cc
cc
pp
pp
88
88
}
The variables dscp1 through dscp8 represent the DSCP values that you would enter. The
seven positions shown correspond to the seven nonzero CoS values.
Because there are only eight CoS values, it is easy to map CoS to DSCP, but slightly more
complicated when you map DSCP to CoS, because it’s not possible to map all 64 DSCP
values to an individual CoS value. As such, groups of DSCP values must be mapped to a
single CoS value. On the Catalyst 6500 Family of switches, that configuration is as follows:
ss
ss
ee
ee
tt
tt
qq
qq
oo
oo
ss
ss
dd
dd
ss
ss
cc
cc
pp
pp
--
--
cc
cc
oo
oo
ss
ss
--
--
mm
mm
aa
aa
pp
pp
22
22
00
00
--
--
22
22
55
55
::
::
77
77
33
33
33
33
--
--
33
33
88
88
::
::
33
33
This example maps DSCP values 20 to 25 to CoS value 7 and DSCP values 33 to 38 to CoS
value 3. It is a little confusing at first, but makes more sense as you become familiar with
the syntax.
There are also default mappings in most switching platforms, but you should consult the
chapters in this book that deal directly with the platforms that you are working with to
determine what those defaults are and whether they are suitable for your network.
A General View of QoS on the Catalyst Platforms
This chapter mostly focuses on very detailed descriptions of the exact functionality of QoS
on specific platforms. This section, however, provides a general overview of QoS concepts
and a glimpse into the material covered in the later chapters. This section purposely avoids
details about the operation of specific mechanisms on Cisco Catalyst switches. Instead, this
section introduces some ideas that you will learn about in more detail in later chapters. The
later chapters discuss the following QoS concepts for each Catalyst switching platform in
more detail than they are introduced here:
• Classification
• Marking
• Policing
• Congestion Management
• Congestion Avoidance
Not all of Catalyst switching families support all the listed features, and those features are
omitted from discussion on nonapplicable switch families. The following sections briefly
discuss these features and their relation to Catalyst switches.
60 Chapter 2: End-to-End QoS: Quality of Service at Layer 3 and Layer 2
Catalyst QoS Classification
Classification determines how a switch or router marks, processes, and schedules frames.
Currently shipping Catalyst switches utilize an internal DSCP value to correctly schedule and
mark frames for egress transmission. Chapter 6 first introduces the concept of internal DSCP.
Moreover, Catalyst switches classify frames based on a variety of ingress frame parameters
such as CoS, DSCP, IP precedence, trust, ingress interface, or IP address. Trust, based either
on platform-specific default or user configuration, is an indication of whether the network
administrator trusts the QoS markings of ingress frames on a per-interface basis. Generally,
network administrators do not trust user ports because operating systems enable users to set
the CoS value on egress packets. This situation may yield a negative impact on the network
because users determine the priority of their traffic. Conversely, network designs typically
trust infrastructure connections such as switch-to-switch, switch-to-router, or switch-to-IP
Phone connections. The assumption in this case is that the other switches are already
configured properly for trusting, classification, and marking.
Trusted interfaces do not alter the QoS marking of ingress frames. Untrusted interfaces alter
the QoS markings to a configurable value CoS or DSCP. This value is typically zero (Best
Effort) for untrusted interfaces. The command-line configuration and restrictions
associated with trusted or untrusted interfaces are per platform. Later chapters discuss these
configurations and restrictions, when the focus is on platform-specific properties. This trust
concept is discussed in more detail in the section “Cisco Catalyst QoS Trust Concept” later
in this chapter.
Catalyst QoS Marking
Marking is the act of a switch or router rewriting the CoS, DSCP, or IP precedence fields
of frames based on classification. Marking modifies the intended ingress frame behavior as
set by the originating device. Catalyst switches use interface configurations or policers to
define marking parameters.
Catalyst QoS Policing
Catalyst switches define policers for applying bandwidth limits to ingress and egress traffic.
In addition, Catalyst switches use policers to mark traffic. For example, it is possible to
configure a policing policy such that all traffic of a certain type below 1 Mbps is marked
with a DSCP value of af31 and all traffic above 1 Mbps is marked with a DSCP value of af32.
As with several with Catalyst QoS features, the configuration syntax and actual behavior
for policing on Catalyst platforms is platform-specific and is discussed in later chapters.
However, common to all platforms is the use of a token bucket concept for the policing
bandwidth function.
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