Cisco 6500 Series Software Configuration Manual page 412

Understanding How PFC QoS Works
•
•
All port types have a low-priority and a high-priority standard transmit queue. 1p3q1t ports have a
medium-priority standard transmit queue. 1p2q2t, 1p3q1t and 1p2q1t ports have a strict-priority
transmit queue in addition to the standard queues.
On 2q2t ports, the default PFC QoS configuration allocates a minimum of 80 percent of the total
transmit queue size to the low-priority standard queue and a minimum of 20 percent to the high-priority
standard queue.
On 1p2q2t, 1p3q1t, and 1p2q1t ports, the switch services traffic in the strict-priority queue before
servicing the standard queues. When the switch is servicing a standard queue, after transmitting a packet,
it 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.
On 1p2q2t ports, the default PFC QoS configuration allocates a minimum of 70 percent of the total
transmit queue size to the low-priority standard queue, a minimum of 15 percent to the high-priority
standard queue, and a minimum of 15 percent to the strict-priority queue.
On 1p3q1t ports, the transmit queue size is not configurable and is allocated equally among all queues.
On 1p2q1t ports, the default PFC QoS configuration allocates a minimum of 50 percent of the total
transmit queue size to the low-priority standard queue, a minimum of 30 percent to the high-priority
standard queue, and a minimum of 20 percent to the strict-priority queue.
Transmit-queue size is limited to the configured value (see the
Note
on a 1p1q0t or 1p1q8t Ingress LAN Ports" section on page
bandwidth (bandwidth is only available when there is no traffic in the other queues).
Scheduling and Congestion Avoidance
These sections describe scheduling and congestion avoidance:
•
•
•
•
Note The explanations in these sections use default values. You can configure many of the parameters (for
more information, see the
use the same drop-threshold configuration.
Catalyst 6500 Series Switch Cisco IOS Software Configuration Guide—Release 12.1 E
31-22
1p3q1t indicates one strict-priority queue and three standard queues, each with one threshold
configurable as either WRED-drop or tail-drop, and one nonconfigurable tail-drop threshold.
1p2q1t indicates one strict-priority queue and two standard queues, each with one configurable
WRED-drop threshold and one nonconfigurable tail-drop threshold.
2q2t Ports, page 31-23
1p2q2t Ports, page 31-23
1p3q1t Ports, page 31-23
1p2q1t Ports, page 31-24
"Configuring PFC QoS" section on page
Chapter 31 Configuring PFC QoS
"Setting the Receive-Queue Size Ratio
31-64), but any queue can use all available
31-33). All ports of the same type
78-14099-04