Enterprise QoS Solution Reference Network Design Guide

Catalyst 6500 PFC2/PFC3—QoS Considerations and Design
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Enterprise QoS Solution Reference Network Design Guide
Chapter 2 Campus QoS Design
1P2Q2T—Indicates one strict-priority queue and two standard queues, each with two configurable
WRED-drop thresholds.
1P3Q1T—Indicates one strict-priority queue and three standard queues, each with one configurable
WRED-drop threshold (however, each standard queue also has one nonconfigurable tail-drop
threshold).
1P3Q8T—Indicates one strict-priority queue and three standard queues, each with eight
configurable WRED-drop thresholds (however, each standard queue also has one nonconfigurable
tail-drop threshold).
1P7Q8T—Indicates one strict-priority queue and seven standard queues, each with eight
configurable WRED-drop thresholds (on 1p7q8t ports, each standard queue also has one
nonconfigurable tail-drop threshold).
Almost all Catalyst 6500 linecards support a strict-priority queue and when supported, the switch
services traffic in the strict-priority transmit 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.
Additionally, Catalyst 6500 linecards implement CoS-value-based transmit-queue drop thresholds to
avoid congestion in transmitted traffic. WRED thresholds can also be defined on certain linecards, where
the CoS value of the packet (not the IP Precedence value, although they likely match) determines the
WRED weight. WRED parameters include a lower and upper threshold: the low WRED threshold is the
queue level where (assigned) traffic begins to be selectively-dropped and the high WRED threshold is
the queue level above which all (assigned) traffic is dropped. Furthermore, packets in the queue between
the low and high WRED thresholds have an increasing chance of being dropped as the queue fills.
The Transmit Queuing/Dropping capabilities can be returned by using the following commands.
CatOS:
– show port capabilities
– show port qos
– show qos info
IOS:
– show queueing interface
Table 2-5 shows the Catalyst 6500 linecards that are currently available and their
respective queuing/dropping structures.
Table 2-5 Catalyst 6500 Linecard Queuing Structures
C2 (xCEF720) Modules Description
WS-X6704-10GE Catalyst 6500 4-port 10 GigabitEthernet
Module
WS-X6724-SFP Catalyst 6500 24-port GigabitEthernet
SFP Module
Receive Queue
Structure
1Q8T
(8Q8T with
DFC3a)
1Q8T;
(2Q8T with
DFC3a)
Transmit Queue
Structure Buffer Size
1P7Q8T 16MB per port
1P3Q8T 1MB per port
Version 3.3