Enterprise QoS Solution Reference Network Design Guide

Chapter 2 Campus QoS Design
Version 3.3
Catalyst 6500 PFC2/PFC3—QoS Considerations and Design
Catalyst 6500—Untrusted Server with Scavenger-Class QoS Model
This section includes the following topics:
Configuration
Catalyst 6500 CatOS QoS Verification Commands
Additional flexibility is offered to the Untrusted Server + Scavenger Model due to the Catalyst 6500
PFC2/PFC3’s support of Dual-Rate Policing (as described in RFC 2698 “A Two Rate Three Color
Marker” and as illustrated in Figure 4-5).
Using a Dual-Rate policer, three colors are used to indicate:
Conforming traffic (within the normal rate)
Excess traffic (exceeding the normal rate but less than the excess rate)
Violating traffic (exceeding both the normal and excess rate)
The Dual-Rate policer is intended to complement the RFC 2597 Assured Forwarding Groups Diff-Serv
marking scheme. To illustrate, consider Transactional Data traffic, which is marked to AF Class 2.
Conforming Transactional Data should be marked to AF21, Excess Transactional Data traffic should be
marked-down to AF22, and Violating Transactional Data traffic should be marked-down further to AF23.
Such a markdown scheme is intended to be complemented further by DSCP-based WRED congestion
avoidance. In this manner, in the event of congestion, AF23 is dropped more aggressively than AF22
which, in turn, is dropped more aggressively than AF21.
However, since Catalyst 6500 queuing and congestion-avoidance is determined primary by CoS
markings, the standards-based DSCP model cannot be followed completely at this time on this platform
(since AF21/AF22/AF23 all share CoS 3, this does not allow for granular sub-class QoS). Therefore, the
use of Scavenger class markings for violating traffic could be used to achieve a similar overall effect,
while maintaining consistency with QoS designs previously presented for other Catalyst platforms.
Under such a modified Untrusted Multi-Application Server with Scavenger-Class QoS Model, Excess
Transactional Data traffic can be marked down to AF22 and Violating Transactional Data traffic can be
marked down to DSCP CS1 (Scavenger). Similarly, Excess Bulk Data traffic can be marked down to
AF12 and Violating Bulk Data traffic can be marked down to DSCP CS1 (Scavenger). This modified
model is shown in Figure 2-24.
Enterprise QoS Solution Reference Network Design Guide
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