Enterprise QoS Solution Reference Network Design Guide

Chapter 2 Campus QoS Design
Catalyst 6500—Queuing and Dropping
Version 3.3
This section includes the following topics:
Catalyst 6500 Queuing and Dropping Overview
Catalyst 6500 Transmit Queuing and Dropping Linecard Options
Catalyst 6500—2Q2T Queuing and Dropping
Catalyst 6500—1P2Q1T Queuing and Dropping
Catalyst 6500—1P2Q2T Queuing and Dropping
Catalyst 6500—1P3Q1T Queuing and Dropping
Catalyst 6500—1P3Q8T Queuing and Dropping
Catalyst 6500—1P7Q8T Queuing and Dropping
Catalyst 6500 Queuing and Dropping Overview
Catalyst 6500 PFC2/PFC3—QoS Considerations and Design
While the Catalyst 6500 PFC performs classification, marking, mapping, and policing functions, all
queuing and congestion avoidance policies are administered by the Catalyst 6500 linecards. This
inevitably leads to per-linecard hardware-specific capabilities and syntax when it comes to configuring
queuing and dropping.
As previously discussed in relation to other platforms that support ingress queuing, receive queues are
extremely difficult to congest, even in controlled lab environments. This is especially so if access edge
policies, as detailed in this chapter, are used on all access layer switches.
Ingress congestion implies that the combined ingress rates of traffic exceed the switch fabric channel
speed, and thus would need to be queued simply to gain access to the switching fabric. On newer
platforms, such as the Catalyst 6500 Sup720, this means that a combined ingress rate of up to 40 Gbps
per slot would be required to create such an event.
However, to obviate such an extreme event, the Catalyst 6500 schedules ingress traffic through the
receive queues based on CoS values. In the default configuration, the scheduler assigns all traffic with
CoS 5 to the strict-priority queue (if present); in the absence of a strict priority queue, the scheduler
assigns all traffic to the standard queues. All other traffic is assigned to the standard queue(s) (with
higher CoS values being assigned preference over lower CoS values, wherever supported). Additionally,
if a port is configured to trust CoS, then the ingress scheduler implements CoS-value-based
receive-queue drop thresholds to avoid congestion in received traffic. Thus, even if the extremely
unlikely event of ingress congestion should occur, the default settings for the Catalyst 6500 linecard
receive queues are more than adequate to protect VoIP and network control traffic.
Therefore, the focus of this section is on Catalyst 6500 egress/transmit queuing design
recommendations.
Catalyst 6500 Transmit Queuing and Dropping Linecard Options
There are currently six main transmit queuing/dropping options for Catalyst 6500 linecards:
2Q2T—Indicates two standard queues, each with two configurable tail-drop thresholds.
1P2Q1T—Indicates one strict-priority queue and two standard queues, each with one configurable
WRED-drop threshold (however, each standard queue also has one nonconfigurable tail-drop
threshold).
Enterprise QoS Solution Reference Network Design Guide
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