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586 C
HAPTER
22: Q
O
S
AND
RSVP
After you specify how the rate limit is expressed, you can specify a burst
size. The
burst size is the maximum amount of data that you can transmit
at the line rate before the transmission is policed. This value
accommodates variations in speeds and allows you to occasionally exceed
the configured rate.
Specifying Service
Levels
When you define a control, you specify a service level (a transmit priority).
Most of the service levels that you can specify represent a specific
transmit queue. You can assign service levels to conforming packets
(packets that are within the rate limit) and to nonconforming excess
packets (packets that exceed the rate limit).
For information on assigning an IEEE 802.1p priority to nonconforming
excess packets, see “QoS Excess Tagging” later in this chapter. For
information on the transmit queues and QoS bandwidth, see “Transmit
Queues and QoS Bandwidth” later in this chapter.
Service levels also define the loss-eligibility status for conforming and
nonconforming excess. By default, conforming packets are
not
loss-eligible; nonconforming excess are loss-eligible.
The Multilayer Switching Module supports these service levels:
■
High — For any type of rate limit, transmits the packet first (top
priority)
■
Best — For any type of rate limit, transmits the packet on a best-effort
basis (the default for conforming and nonconforming excess packets)
■
Low — For any type of rate limit, transmits the packet on a
low-priority basis
■
Drop — For a rate limit of none, drops all conforming packets on all
ports associated with the classifiers. For a rate limit of receivePort or
aggregate, drops all nonconforming excess packets.
If you want to drop conforming packets for only a subset of ports, use
the receivePort or aggregate rate limit, set the rate limit to 0, and
specify the group of ports.
If you specify drop for the service level for conforming packets (that is,
you are using a rate limit of none), the Multilayer Switching Module
does not give you the option of specifying an IEEE 802.1p tag.