Last Updated on August 7, 2021 by Admin 3
You issue the show mls qos maps cosdscp command and receive the following output:
An untagged packet arrives on a CoStrusted port. The port is using the default CoS settings.
Which of the following internal DSCP values will the switch use for the packet? (Select the best answer.)
The switch will use an internal Differentiated Services Code Point (DSCP) value of 0 for the packet. The Class of Service (CoS)toDSCP 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. The CoStoDSCP map in this scenario indicates that a CoS value of 0 corresponds to a DSCP value of 0.
The CoStoDSCP map in this scenario is the default CoStoDSCP map. You can modify the CoStoDSCP map by issuing the mls qos map cosdscp dscp1 dscp2 …dscp8 command. The no mls qos map cosdscp command causes the switch to use the default CoStoDSCP map.
The CoS field is a Quality of Service (QoS) 3bit marking field, whereas the DSCP is a QoS 6bit marking field. The following table shows the relationship between CoS and DSCP values:
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.
If the packet in this scenario had a CoS of 1, or if the default CoS for the port were set to a value of 1, the switch would have used an internal DSCP value of 8. If the packet in this scenario had a CoS of 3, or if the default CoS for the port were set to a value of 3, the switch would have used an internal DSCP value of 24. If the packet in this scenario had a CoS of 7, or if the default CoS for the port were set to a value of 7, the switch would have used an internal DSCP value of 56.
The DSCP value 46 corresponds to a special classification known as DSCP Expedited Forwarding (EF). DSCP EF, which is defined in Request for Comments (RFC) 2598, indicates a highpriority packet that should be given queuing priority over other packets but should not be allowed to completely monopolize the interface. Voice over IP (VoIP) traffic is often assigned to DSCP EF.