Juniper JUNOSE 11.1.X - QUALITY OF SERVICE CONFIGURATION GUIDE 3-21-2010, Руководство по настройке

Страница: 97 / 398

whereas competing queues must leave the active WRR because their weight credits
are exhausted. To completely drain the queue, configure the maximum burst size.
The zero-weighted queue is eventually alone in the active round robin and is effectively
drained at strict priority.
To configure more than one relative strict queue or node, simply configure a
maximum weight, and the two relative strict queues or nodes will share bandwidth
fairly. You can shape the nonstrict queue, as described in the next section, to keep
latency bounded.
Also, configure only a few nonstrict nodes or queues to prevent additional latency
and jitter of the relative strict-priority traffic when the nodes or queues are in the
round robin and a packet arrives in the zero-weighted queue. The number of nonstrict
frames that precede a relative strict frame equals the number of nonzero weighted
queues among the sibling scheduler nodes.
Nonstrict queues must still exhaust their weight credits before they leave the active
round robin. The result is that occasionally more than one nonstrict frame may
precede a relative strict frame, causing more jitter than may be acceptable. You can
eliminate this source of latency by shaping the nonstrict queue to the aggregate rate
with a burst size of 1.
Setting the Burst Size in a Shaping Rate
The burst value in a shaping rate determines the number of rate credits that can
accrue when the queue or scheduler node is held in the inactive round robin. When
the queue is back on the active list, the accrued credits allow the queue or node to
catch up to the configured rate, up to the burst value.
Normally, the burst size is several packet lengths to allow a queue deprived of
bandwidth because of congestion to catch up to its rate. Larger burst sizes allow
more bursting to allow the queue to attain its shaped rate under bursty congestion
scenarios.
Special Shaping Rate for Nonstrict Queues
To remove additional jitter, you can configure the nonstrict queue with a special
shaping rate that causes the hardware to temporarily eject the queue from the active
round robin whenever it sends a frame. The result is that at most one nonstrict frame
can precede a relative strict-priority frame. The special shaping rate is the same rate
as the aggregate rate, but with a configured burst size of 1.
You can still configure a shaping rate for the zero-weighted queue or node. This is
useful for limiting starvation of the nonstrict traffic in the aggregate.
In Figure 14 on page 66, the VC node is shaped in the HRR scheduler to 1 Mbps to
limit the aggregate traffic for the subscriber. The relative strict traffic is shaped to
500 Kbps. This shaping limits relative strict traffic to 500 Kbps, and prevents the
relative strict-priority traffic from starving out the nonstrict traffic.
The third shaper, on the nonstrict queue, is subtle. The rate is 1 Mbps, which allows
the nonstrict traffic to consume up to the full aggregate rate of the VC. But the burst
size is 1, which causes the nonstrict queue to always yield to the relative strict-priority
Comparison of True Strict Priority with Relative Strict Priority Scheduling
■ 65
Chapter 8: Configuring Strict-Priority Scheduling