31.0 Class of Service (CoS):
When frames (layer 2) carried from one switch boundary to another switch boundary, prioritization of traffic can be achieved by utilizing the Class of Service field (CoS) of the frame tag. For prioritization of traffic, both 802.1Q, and ISL provide a field to represent CoS of each frame. The value 0 of the CoS field indicates lowest priority, and the value 7 indicates the highest priority frame. CoS information is passed along ISL and 802.1Q trunks.
32.0 DSCP
Codepoint Name | DSCP Bits | Binary Decimal |
---|---|---|
Default | 000 000 | 0 |
AF11 | 001 010 | 10 |
AF12 | 001 100 | 12 |
AF13 | 001 110 | 14 |
AF21 | 010 010 | 18 |
AF22 | 010 100 | 20 |
AF23 | 010 110 | 22 |
AF31 | 011 010 | 26 |
AF32 | 011 100 | 28 |
AF32 | 011 110 | 30 |
AF41 | 100 010 | 34 |
AF42 | 100 100 | 36 |
AF43 | 100 110 | 38 |
EF | 101 110 | 46 |
The best service available to user traffic is represented by DSCP Codepoint name EF. EF has no class or drop precedence notation.
33.0 To determine the Transmit or Egress queue types available
on a Catalyst 6500, use the command:
Show queueing interface <type> <mod>/<num>
Ex: show queueing interface gigabitethernet 1/1
For IOS based
Catalyst switches, use the command:
Show interfaces <type> <mod>/<num>
capabilities
Ex: show interfaces gigabitethernet 0/1 capabilities
34.0 QoS:
In a Catalyst switch, the QoS is disabled by default. To enable
QoS on a switch, use the command:
SwitchA(config)#mls qos
QoS policy is applied to an interface. You can apply one for inbound traffic, and another for outbound traffic on the same interface.
The inbound QoS information is mapped using the following commands
as appropriate:
CoS: CoS values (0 to 7) are mapped into an internal
DSCP value (0 to 63)
Switch(config)# mls qos map cos-dscp dscp1
dscp2 ... dscp8
Ip Precedence: IP Precedence values ( 0 to 7)
are mapped into an internal DSCP value (0 to 63).
Switch(config)#
mls qos map ip-prec-dscp dscp1 dscp2 ... dscp8
DSCP: Inbound DSCP values can be mapped into internal DSCP values
(which are different from that of inbound DSCP values). To enable
DSCP mutation, you need to first create the map consisting of up
to eight entries by using the global configuration command:
Switch(config)#
mls qos map dscp-mutation <dscp-mutation-name> <in-dscp> to <out-dscp>
<in-dscp>
and <out-dscp> are values within 0 to 63.
Ex: switch(config)#
mls qos map dscp-mutation mydscpmap 18 to 21
Use the above
command as many times as required. Then apply the mutation map to
a specific ingress interface by using the interface configuration
command:
Switch(config-if)# mls qos dscp-mutation <dscp-mutation-name>
Ex: switch(config-if)# mls qos dscp-mutation mydscpmap
The sequence of steps involved in defining a QoS policy on a
switch is as below:
1. Define one or more QoS classes. QoS
classes are used to identify specific traffic. Use the command:
Switch(config)# class-map <class-name> [match-all | match-any]
2. Define one or more QoS policies. QoS policy is used to reference
or group multiple QoS classes as a single entity. Use the command:
Switch(config)# policy-map <policy-name>
In the policy
map, use the command:
Switch(config-pmap)# class <class-name>
to identify each class map that will be used as part of the overall
QoS policy.
3. Assign one QoS policy to an interface. An
interface can have only one active policy applied in each direction.
You can have different policies for inbound and outbound traffic
on the same interface. Use the command:
Switch(cofig-if)#
service-policy [input | output] <policy-name>
to apply
the policy to inbound or outbound traffic.
The strict-priority queue on a switch interface has to be enabled
before being used. The interface configuration mode command for
enabling a strict-priority queue on an interface is:
Switch(config-if)#
priority-queue out
Commonly used commands that display the QoS information on a
Catalyst switch are given below:
1. show mls qos interface <type> <mod/num>
; displays QoS trust configured on an interface.
2. show mls
qos interface <type> <mod/num> queueing ; You can verify the
status of egress queueing on an interface.
3. show queueing interface <type> <mod>/<num>
; Used to verify QoS settings only on a Catalyst 6500 interface.
4. show mls qos maps ; View all QoS parameter mappings.
The correct syntax for configuring the QoS trust on the data
port of an IP Phone is:
switch(config-if)# switchport priority
extend {cos value | trust}
The switch instructs an attached
IP Phone through CDP messages as to how it should extend QoS trust
to its PC data switch port. By default, a switch instructs an attached
IP Phone not to trust the PC port. CoS values are overwritten to
0.
35.0 Traffic Shaping:
Traffic shaping is generally used for limiting the overall traffic. VOIP is a time sensitive traffic, and generic traffic shaping may not be suitable for VOIP traffic on a switch interface. Traffic policing is sensitive to the type of traffic (such as VOIP or any time critical traffic) and is suitable for use on a switch. GTS and AGTS are traffic shaping method used on Cisco routers.
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