Enterprise QoS Solution Reference Network Design Guide

Catalyst 2950—QoS Considerations and Design
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Enterprise QoS Solution Reference Network Design Guide
Chapter 2 Campus QoS Design
Note Alternatively, as these weights are relative, they can be reduced by dividing each weight by the lowest
common denominator (in this case 5) to arrive at queue weights of 1, 5 and 14 for Queues 1, 2 and 3
(respectively). Reduction is strictly optional and makes no difference to the servicing of the queues.
Many network administrators tend to prefer defining bandwidth allocation ratios as percentages, and so
bandwidth weight ratios aren’t reduced in this design chapter.
So far, Campus QoS designs have been presented for the first half of the DoS/worm mitigation strategy
discussed at the beginning of this chapter, namely, designs for access layer policers to mark down
out-of-profile traffic to the Scavenger class PHB of CS1.
The second vital component of this strategy is to map Scavenger class traffic into a “less-than
Best-Effort” queuing structure, ensuring that all other traffic will be serviced ahead of it in the event of
congestion.
The Catalyst 2950, like most Catalyst platforms, supports the mapping of CoS values into queues. The
CoS value that corresponds to Scavenger (DSCP CS1) is CoS 1; this CoS value is shared with Bulk data
(DSCP AF11). Therefore, a small amount of bandwidth (5%) is allocated to the “less-than Best-Effort”
queue: Q1. Q1 will thus service legitimate Bulk traffic but will constrain out-of-profile Scavenger
traffic—which may be the result of a DoS/worm attack—to a small amount (