Cisco WS-X6324-100FX-MM White Paper - Page 6
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The threshold is also used to associate itself with certain packets tagged with a priority value. When the threshold has been exceeded, the WRED
process will attempt to drop packets associated with that breached threshold level. The network administrator also has the ability to set the threshold
level.
Ingress and Egress QoS: Maps
The Cisco Catalyst 6500 uses maps in a number of forms to perform different duties. The following section summarizes the different maps available.
Maps: Mapping a Packet to a Queue or Threshold
Given that a switch port can have multiple queues, a way must be provided for the switch to determine in which queue a given piece of data should
be placed. The way in which this is determined is by using a map. A map sets out a two-column table. The first column contains the priority value
that would be found in the packet's header. The second column contains the queue (and threshold in that queue) to which the packet should be
assigned. This is shown in Figure 6.
Figure 6. Mapping a Packet to a Queue or Threshold
Maps: Mapping Priority Values
When a packet arrives at a switch port, it can be primed with a priority value. The trust setting of the port will determine which priority setting (ToS
or CoS) will be honored by the switch. As the packet passes through the switch (that is, after it has arrived on the input port and prior to it being sent
out the output port), it is assigned a priority value that is only used internal to the switch. This internal priority value is referred to as the internal
DSCP. A map is used to derive the internal DSCP from the incoming packet's priority setting. After the packet has passed through the switch,
another map is used to derive what the CoS value will be written as for the packet when it is transmitted out the switch port. A summary of these
maps is presented in Table 1.
Table 1.
Map Summary
Map Name
CoS to DSCP Map
IP Precedence to DSCP Map
DSCP to CoS Map
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Related Trust Setting
Used on Input or Output
Trust CoS
Input
Trust IP precedence
Input
–
Output
© 2005 Cisco Systems, Inc. All rights reserved.
Map Description
Derives the internal DSCP from the incoming CoS value
Derives the internal DSCP from the incoming IP
precedence value
Derives the CoS for the outbound packet from the internal
DSCP
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