Enterprise QoS Solution Reference Network Design Guide

Chapter 2 Campus QoS Design
Catalyst 6500—1P2Q1T Queuing and Dropping
Version 3.3
Catalyst 6500 PFC2/PFC3—QoS Considerations and Design
This section includes the following topics:
Configuration
Catalyst MLS QoS Verification Commands
The 1P2Q1T queuing model builds on the previous 2Q2T model, bringing with it the advantages of
strict-priority queuing (for VoIP) as well as a tunable WRED (not Tail-Drop) threshold per queue.
The term “1P2Q1T” is a bit of a misnomer in the CatOS version of this queuing structure because in
CatOS there are actually two thresholds per queue: the tunable WRED threshold and the
non-configurable tail-of-the-queue (100%) tail-drop threshold.
Under such a model, buffer space can be allocated as follows: 30% for Scavenger/Bulk with Best Effort
queue (Q1), 40% for Q2, and 30% for the PQ (Q3)
The WRR weights for Q1 and Q2 (for dividing the remaining bandwidth, after the priority queue has
been fully serviced) can be set to 30:70 respectively for Q1:Q2.
Under the 1P2Q1T model, each queue’s WRED threshold is defined with a lower and upper limit. For
example, the WRED threshold 40:80 indicates that packets assigned to this WRED threshold will begin
being randomly dropped when the queue fills to 40 percent and that these packets will be tail-dropped
if the queue fills beyond 80 percent.
Furthermore, in CatOS within the 1P2Q1T queuing structure, each CoS value can be assigned to queue
and a WRED threshold or just to a queue. When assigned to a queue (only), then the CoS value is limited
only by the tail of the queue (in other words, it is assigned to the queue with a tail-drop threshold of
100%).
Thus (in CatOS), the tunable WRED threshold for Q1 can be set to 40:80, meaning that Scavenger/Bulk
Data will be WRED-dropped if Q1 fills to 40 percent and will be tail-dropped if Q1 fills past 80 percent
of capacity. This prevents Scavenger/Bulk Data from drowning out Best-Effort traffic in Q1. The WRED
threshold for Q2 can be set to 70:80 to provide congestion avoidance for all applications assigned to it
and to ensure that there will always be room in the queue to service Network and Internetwork Control
traffic.
Therefore, once the queues and thresholds have been defined as above, then CoS 1 (Scavenger/Bulk) can
be assigned to Q1T1; CoS 0 (Best Effort) to Q1-only (tail); CoS 2 (Network Management and
Transactional Data), CoS 3 (call signaling and Mission-Critical Data) and CoS 4 (Interactive and
Streaming Video) can be assigned to Q2T1; CoS 6 and 7 (Internetwork and Network Control) can be
assigned to Q2-only (tail); CoS 5 (VoIP) can be assigned to Q3 (the PQ).
These 1P2Q1T queuing recommendations are illustrated in Figure 2-27.
Enterprise QoS Solution Reference Network Design Guide
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