Specifications

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7.13 Egress DSCP Mutation Mapping
The switch will derive an internal DSCP value from the incoming packet based on the trust setting of the port.
Assuming ToS Byte preservation is not configured, this internal DSCP value is used to write the ToS value on the
egress packet. Egress DSCP Mutation is a feature that allows this internal DSCP value to be changed, and the
changed value be used to as the priority setting for the egress packet.
All versions of the PFC3 now support egress DSCP mutation maps that allow a set of DSCP values to be mutated
(or changed) to a set value. Up to 15 maps can be defined and within each map, up to 8 DSCP values can be
associated with a mutated value. An egress DSCP mutation map is configured as follows:
Cat6500(config)# mls qos dscp-mutation mapname 30 31 32 to 16
In the example above, the mutation map called “mapname” is applied to a given interface, and any packet with an
internal DSCP setting of 30, 31 or 32 will have the DSCP value of 16 written into the packet prior to being
transmitted.
7.14 DSCP to CoS Mapping
When the frame has been placed into the egress port, the port asic will use the assigned CoS to perform congestion
avoidance (i.e. WRED) and also use the CoS to determine the scheduling of the frame (i.e. transmitting the frame).
At this point the switch will use a default map to take the assigned DSCP and map that back to a CoS value. This
default map is displayed in Table 3.
Alternatively, the administrator can create a map that will be used by the switch to take the assigned internal DSCP
value and create a new CoS value for the frame. Examples of how you would use CatOS and Cisco IOS to achieve
this are shown below.
Cat6500(config)# mls qos map dscp-cos 20 30 40 50 52 10 1 to 3
This sets DSCP values of 20, 30, 40, 50, 52, 10 and 1 to a CoS value of 3.
7.15 Adjusting the Transmit Queue Size Ratio
Each port has a set amount of buffer space allocated to it to store transient data. The amount of buffer space
allocated between transmit queues on that port can be adjusted to meet set requirements of data residing in a given
queue. This capability uses a percentage as a way for the administrator to define the allocated buffer space to a
given transmit queue.
Figure 30. Transmit Queue Size Ratio
The command to change the ratio of buffer space on the transmit queues is defined as follows:
Cat6500(config)# wrr-queue queue-limit 30 30 20