Specifying Service Levels - 3Com corebuilder 3500 Implementation Manual

Specifying Service
Levels
After specifying 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.
When you define a control, you specify a service level (a transmit priority).
Most of the service levels you can specify represent a specific transmit
queue. You can assign service levels to conforming packets (packets that
are within the rate limit) and 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 system 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 only, drops all conforming packets on
all ports associated with the classifier(s). 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 (you
are using a rate limit of none), the system does not give you the
option of specifying an IEEE 802.1p tag.
QoS Controls 493