642-642 QOS VIDEO TRAINING: Implementing QoS on Catalyst Switches

Implementing QoS on Catalyst Switches


1. Allows traffic to be classified and marked as close to the source as possible.

2. Policing performed as close to the source as possible (and more options)

3. Hardware vs Software switches (Router perform software queuing – RAID analogy, switches perform QoS using ASIC hardware)

How QoS is done with the Catalyst

QoS is hardware based with ASIC

Switching capabilities are defined in terms of:

  • Queues – Priority or Standard
  • Thresholds – How many packets can be help in the queue – Taildrop or RED/WRED to assist when limit has exceeded.

Examples – Standards

  • 2Q2T – 2 queues and 2 thresholds per queue (CoS)
  • IP2Q2T – 1 Priority Queue followed by 2 seperate queues with 2 drop thresholds per queue.

What can you do with the queues?

It depends on your hardware platform and how you configure them.

Queues can be configured with:

  • Priority Queuing
  • Weighted Round Robin – (Custom Queuing equivalent)
  • WRR with Priority Queuing

Thresholds can be configured with:

  • Taildrop
  • WRED

How the switches line up…

Model: 2950

Transmit Queues: 1P3Q, 4Q (WRR)

Model: 3550

Transmit Queues: 1P3Q2T, 4Q2T

Model: 3750

Transmit Queues: 4Q3T

Model: 4000

Transmit Queues: 1P3Q2T, 4Q2T

Model: 6500

Transmit Queues: 2Q2T, 1P2Q2T, 1P3Q1T, 1P2Q1T

3550 Configuration Examples

Class Map/Policy Map and Service Policy can still be used (MQC) However, limited commands compared to a router. For example NBAR is toned down.

*Cisco phones mark traffic themselves.

mls qos – enables QoS globally on a Catalyst switch

Populates cfg into the running cfg


CoS to DSCP mappings Layer 2 to Layer 3 standard values populated.

mls qos map ? offers DSCP to CoS for example…

min-reserve = number of reserved buffers

Interface Configuration

int Fa0/1

mls qos trust ? cos, device, dscp, ip-precendence


Specific to certain Catalyst switches…

What values go into what queues?

All focused around CoS

wrr-queue cos-map (1-4) Each CoS value assigned to queue

wrr-queue cos-map 1 0 1 2 – CoS values in Q1

Bandwidth setting for WRR:

wrr-queue bandwidth (Q1) 5 (Q2) 6 etc….

Bandwidth setting is weird… :s more of a percentage. More homework!

priority-queue out – 3550 specific!