Exam 400-101 Question id=741 Infrastructure Services

Which of the following commands is functionally equivalent to the no mls qos map cos-dscp command, thereby setting the CoStoDSCP map to the default settings?

A. mls qos map cos-dscp 0 4 8 12 16 20 24 28
B. mls qos map cos-dscp 0 8 16 24 32 40 48 56
C. mls qos map cos-dscp 4 8 12 16 20 24 28 32
D. mls qos map cos-dscp 8 16 24 32 40 48 56 64

The mls qos map cosdscp 0 8 16 24 32 40 48 56 command is functionally equivalent to the no mls qos map cosdscp command, thereby setting the Class of Service (CoS) to Differentiated Services Code Point (DSCP) map to the default settings. The CoS field is a Quality of Service (QoS) 3bit marking field, whereas the DSCP is a QoS 6bit marking field.

The first three bits of the DSCP value are the same as the CoS value? the DSCP value just has three extra 0 bits appended to the end. If you know the CoS value of a packet, you can derive the default DSCP value by converting the CoS value to binary, appending three 0 bits, and converting back to decimal.

The CoStoDSCP map is used to generate an internal DSCP value for packets that arrive on a CoStrusted port. By default, the CoS for untagged packets is set to 0, so the internal DSCP value would also be 0. If the CoS were 4, the internal DSCP value used by the switch would be 32. You can view the current CoStoDSCP map by issuing the show mls qos maps cosdscp command.


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