Enterprise QoS Solution Reference Network Design Guide

Catalyst 3550—QoS Considerations and Design
Advanced Tuning Options
2-42
Figure 2-15 Catalyst 3550 1P3Q2T Queuing Model
Application
Voice
Interactive-Video
Streaming-Video
Call-Signaling
Transactional Data
Network-Management
Bulk Data
DSCP
Network Control -
Internetwork Control CS6
Enterprise QoS Solution Reference Network Design Guide
EF
AF41
CS4
Mission-Critical Data DSCP 25
AF31/CS3
AF21
CS2
AF11
Scavenger CS1
Best-Effort 0
CoS
CoS 7
CoS 6
CoS 5
CoS 4
CoS 4
CoS 3
CoS 3
CoS 2
CoS 2
CoS 1
CoS 1
Chapter 2 Campus QoS Design
The interface-mode configuration commands to configure this 1P3Q1T queuing model, for either
FastEthernet or GigabitEthernet Catalyst 3550 interfaces, are shown below.
Example 2-22 Catalyst 3550 FastEthernet and/or GigabitEthernet Interface Queuing
Configuration—1P3Q1T
0
CAT3550(config)#interface range FastEthernet0/1 - 48
CAT3550(config-if)# wrr-queue bandwidth 5 25 70 1 ! Q1-5% Q2-25% Q3-70% Q4=PQ
CAT3550(config-if)# wrr-queue cos-map 1 1 ! Assigns Scavenger to Q1
CAT3550(config-if)# wrr-queue cos-map 2 0 ! Assigns Best Effort to Q2
CAT3550(config-if)# wrr-queue cos-map 3 2 3 4 6 7 ! Assigns CoS 2,3,4,6,7 to Q3
CAT3550(config-if)# wrr-queue cos-map 4 5 ! Assigns VoIP to Q4 (PQ)
CAT3550(config-if)# priority-queue out ! Enables Q4 as PQ
CAT3550(config-if)#exit
CAT3550(config)#
The Catalyst 3550 offers some advanced “nerd-knob” queuing options on 10/100 interfaces, such as
tuning Minimum Reserve Thresholds. However, testing has shown that such tuning, which rarely factors
into play, makes a highly-negligible difference at best. Therefore, tuning Minimum Reserve Thresholds
is recommended only for advanced network administrators or via automated tools, such as AutoQoS.
Some advanced “nerd-knobs” also exist for GigabitEthernet interfaces. A couple of these advanced
tuning options include Queue-Limit tuning and the enabling of WRED thresholds for 1P3Q2T operation.
In the event of DoS/worm attacks, the GigabitEthernet uplinks will likely be the first to congest,
therefore its worthwhile examining these advanced options.
In the example below, the queue-limits for both GigabitEthernet interfaces are tuned to correspond to the
WRR weights of the queues (the bandwidth allocations). This is achieved with the wrr-queue
queue-limit interface command. However, unlike the WRR weight bandwidth ratio for Q4 (which is set
to 1 to indicate that Q4 is a PQ), the queue limit for Q4 needs to be explicitly set to a more representative
value, such as 30%.
CoS 5
CoS 7
CoS 6
CoS 4
CoS 3
CoS 2
CoS 0
CoS 1
1P3Q2T
Q4
Priority Queue
Queue 3
(70%)
Queue 2
(25%)
Queue 1 (5%)
Q3T2
Q3T1
Q1T2
Q1T1
Version 3.3