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5. QoS Flow in the Catalyst 6500

QoS in the Catalyst 6500 is the most comprehensive implementation of QoS found in all of the current Cisco

Catalyst Switches. The following sections will describe how the various QoS processes are applied to a frame as it

transits the switch.

Before continuing further, the flow of QoS though the Catalyst 6500 switch will be reviewed. Earlier it was noted that

there are a number of QOS elements that many layer 2 and layer 3 switches can offer, those being Classification,

Input Queue Scheduling, Policing, Rewriting and Output Queue Scheduling. The following summarizes how the

Catalyst 6500 implements these elements.

Figure 17. QoS Flow on the Catalyst 6500

A frame enters the switch and is initially processed by the port ASIC that received the frame. It will place the frame

into a receive queue.

The port ASIC will use the CoS bits as an indicator of which queue to place the frame into (if more than one input

queue is present). If the port is classified as un-trusted, then the port ASIC can overwrite the existing CoS bits based

on a predefined value.

The frame is then passed to the L2/L3 Forwarding Engine (PFC), which will classify and optionally police (rate limit)

the frame. Classification is the process of assigning the frame a DSCP value (refer to earlier explanation of DSCP),

which is used internally by switch for processing the frame and assigning it a service level. The DSCP will be

derived from one of the following:

1. An existing DSCP value set prior to the frame entering the switch

2. Derived from the received IP Precedence bits already set in the IPv4 header. As there are 64 DSCP values and

only 8 IP Precedence values, the administrator will configure a mapping that is used by the switch to derive the

DSCP. Default mappings are in place should the administrator not configure the maps. Mappings are explained

later in this document.

3. Derived from the received CoS bits already set prior to the frame entering the switch. Like IP Precedence, there

are a maximum of 8 CoS values each of which must be mapped to one of 64.DSCP values. This map can be

configured or the switch can use the default map in place.