Cisco 7604 Configuration Manual page 626

Understanding How PFC QoS Works
The Cisco 7600 series router LAN modules support the following types of scheduling algorithms
between queues:
• Shaped round robin (SRR)—SRR allows a queue to use only the allocated bandwidth.
Deficit weighted round robin (DWRR)—DWRR keeps track of any lower-priority queue
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under-transmission caused by traffic in a higher-priority queue and compensates in the next round.
• Weighted Round Robin (WRR)—WRR does not explicitly reserve bandwidth for the queues. Instead,
the amount of bandwidth assigned to each queue is user configurable. The percentage or weight allocated
to a queue defines the amount of bandwidth allocated to the queue.
Strict-priority queueing—Strict priority queueing allows delay-sensitive data such as voice to be
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dequeued and sent before packets in other queues are dequeued, giving delay-sensitive data preferential
treatment over other traffic. The router services traffic in the strict-priority transmit queue before
servicing the standard queues. After transmitting a packet from a standard queue, the switch checks
for traffic in the strict-priority queue. If the switch detects traffic in the strict-priority queue, it
suspends its service of the standard queue and completes service of all traffic in the strict-priority
queue before returning to the standard queue.
The Cisco 7600 series router LAN modules provides congestion avoidance with these types of
thresholds within a queue:
Weighted Random Early Detection (WRED)—On ports with WRED drop thresholds, frames with a
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given QoS label are admitted to the queue based on a random probability designed to avoid buffer
congestion. The probability of a frame with a given QoS label being admitted to the queue or
discarded depends on the weight and threshold assigned to that QoS label.
For example, if CoS 2 is assigned to queue 1, threshold 2, and the threshold 2 levels are 40 percent
(low) and 80 percent (high), then frames with CoS 2 will not be dropped until queue 1 is at least
40 percent full. As the queue depth approaches 80 percent, frames with CoS 2 have an increasingly
higher probability of being discarded rather than being admitted to the queue. Once the queue is over
80 percent full, all CoS 2 frames are dropped until the queue is less than 80 percent full. The frames
the switch discards when the queue level is between the low and high thresholds are picked out at
random, rather than on a per-flow basis or in a FIFO manner. This method works well with protocols
such as TCP that can adjust to periodic packet drops by backing off and adjusting their transmission
window size.
Tail-drop thresholds—On ports with tail-drop thresholds, frames with a given QoS label are
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admitted to the queue until the drop threshold associated with that QoS label is exceeded;
subsequent frames of that QoS label are discarded until the threshold is no longer exceeded. For
example, if CoS 1 is assigned to queue 1, threshold 2, and the threshold 2 watermark is 60 percent,
then frames with CoS 1 will not be dropped until queue 1 is 60 percent full. All subsequent CoS 1
frames will be dropped until the queue is less than 60 percent full. With some port types, you can
configure the standard receive queue to use both a tail-drop and a WRED-drop threshold by mapping
a CoS value to the queue or to the queue and a threshold. The switch uses the tail-drop threshold for
traffic carrying CoS values mapped only to the queue. The switch uses WRED-drop thresholds for
traffic carrying CoS values mapped to the queue and a threshold. All LAN ports of the same type
use the same drop-threshold configuration.
In Release 12.2(18)SXF5 and later releases, you can enable DSCP-based queues and thresholds on
Note
WS-X6708-10GE ports (see the
The combination of multiple queues and the scheduling algorithms associated with each queue allows the
switch to provide
Cisco 7600 Series Router Cisco IOS Software Configuration Guide, Release 12.2SX
41-24
"Configuring DSCP-Based Queue Mapping" section on page
congestion avoidance.
Chapter 41 Configuring PFC QoS
41-100).
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