3Com 4007, Руководство по внедрению, Страница: 606 / 684

606 C
HAPTER
22: Q
O
S
AND
RSVP
Transmit Queues
and QoS Bandwidth
QoS uses four transmit queues:
■
Control queue — The transmit queue for reserved network control
traffic, such as RIP or OSPF updates, as well as RSVP data flows. This
queue is always serviced first. Bandwidth for this queue is set via RSVP.
■
High priority queue — The transmit queue with the second highest
priority. You can map classifiers directly to this queue.
■
Best effort queue — The transmit queue used by default for all
traffic except reserved traffic.
■
Low priority queue — The transmit queue with the lowest priority.
All traffic assigned to this queue is forwarded only if there is
bandwidth still available after the other queues are serviced. Low
priority packets do not have bandwidth allocated.
You can configure the weighting of the high-priority and best-effort
transmit queues by using the option to modify QoS bandwidth. By
default, the weighting of the queues is 75 percent high-priority traffic
and 25 percent best-effort traffic. Keep in mind that the weighting does
not represent guaranteed output bandwidth for these queues, because
they are served in relative percentages after the control queue is serviced.
When you modify the QoS bandwidth, you specify the percentage of
bandwidth to be used for the high-priority transmit queue on the output
link. You can specify a value in the range from
0 through 100 . The value
that you specify determines the ratio of high-priority to best-effort traffic,
as follows:
■
The value 75 (the default) specifies that three high-priority packets are
transmitted for each best effort packet (ratio of 75/25).
■
The value 50 sets equal priority for high-priority and best-effort
packets (ratio of 50/50).
■
The value
100
is strict prioritization; it allows best effort packets to be
sent only when no high-priority packets need to be sent.
No bandwidth is ever lost. Because QoS uses ratios, any unused
bandwidth can be used by a lower-priority queue.