CCNP Switching Exam Notes: Class Of Service, Dscp, And Qos.

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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