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Catalyst 6500 Series Switch Software Configuration Guide—Release 8.7
OL-8978-04
Chapter 52 Configuring QoS
Understanding How QoS Works
QoS uses the configurable mapping tables to set the DSCP value, which is 6 bits, from the CoS and IP
precedence, which are 3-bit values (for more information, see the
“Mapping the Received CoS Values to
the Internal DSCP Values” section on page 52-73
and the
“Mapping the Received IP Precedence Values
to the Internal DSCP Values” section on page 52-74
).
Policers
You can create the named policers that specify the bandwidth utilization limits, which you can apply to
the traffic by including the policer name in an ACE (for more information, see the
“Creating Policers”
section on page 52-42
).
Policing uses a token bucket scheme. As the packets arrive, the packet size in bytes is added to the bucket
level. Every 0.25 ms, a value equal to the token rate is subtracted from the bucket level.
You specify the bandwidth utilization limits as an average rate and a maximum burst size. The packets
that exceed these limits are “out of profile.” The traffic is in profile as long as it flows in at an average
rate and never bursts beyond the burst size.
With PFC and PFC2, the policing rates use the Layer 3 packet size. With PFC3, the policing rates use
the Layer 2 frame size.
In each policer, you specify if the out-of-profile packets are to be dropped or to have a new DSCP value
applied to them (applying a new DSCP value is called “markdown”). Because the out-of-profile packets
do not retain their original priority, they are not counted as part of the bandwidth that is consumed by
the in-profile packets.
For all policers, QoS uses a configurable table that maps the received DSCP values to the marked-down
DSCP values (for more information, see the
“Mapping the DSCP Markdown Values” section on
page 52-75
). When the markdown occurs, QoS gets the marked-down DSCP value from the table. You
cannot specify a marked-down DSCP value in the individual policers.
Note
By default, the markdown table is configured so that no markdown occurs; the marked-down DSCP
values are equal to the received DSCP values. To enable the markdown, configure the table appropriately
for your network.
You give each policer a unique name when you create it and then use the name to include the policer in
an ACE. The same policer can be used in multiple ACEs.
You can create these policers:
•
Microflow—QoS applies the bandwidth limit that is specified in a microflow policer separately to
each flow that matches any ACEs that use that particular microflow policer. You can create up to
63 microflow policers.
•
Aggregate—QoS applies the bandwidth limits that are specified in an aggregate policer
cumulatively to all flows that match any ACEs that use that particular aggregate policer. You can
create up to 1023 aggregate policers.
•
With PFC2 and PFC3A, you can specify a dual rate aggregate policer with a normal rate and an
excess rate:
–
Normal rate—The packets exceeding this rate are marked down.
–
Excess rate—The packets exceeding this rate are either marked down or dropped as specified
by the drop indication flag.