Cisco 3750 QoS Overview

Cisco 3750 QoS Overview

With QoS, you can provide preferential treatment to certain types of traffic at the expense of others. You can differentiate the traffic using QoS labels. The two most commonly used QoS labels in the Layer 3 IP header are the IP precedence field and the DSCP field. The QoS label in the Layer 2 frame header is called Class of Service (CoS). Catalyst switch QoS tools can provide the preferential treatment based on either Layer 3 QoS labels or Layer 2 QoS labels. This document provides various examples that can give you an idea of the Layer 2 and Layer 3 QoS labels usage in Cisco Catalyst switches.

Cisco Catalyst 3750 Switch without QoS

QoS is disabled by default on the Catalyst 3750 Switches. While QoS is disabled, all frames/packets are passed-through the switch unaltered. For example, if a frame with CoS 5 and the packet inside the frame with DSCP EF enters the switch, the CoS and DSCP labels are not changed. The traffic leaves with the same CoS and DSCP values as it enters. All the traffic, which includes voice, are delivered on the best effort basis.

Cisco Catalyst 3750 Switch QoS Features

After the QoS is enabled on the 3750 Switch, there are few ingress and egress QoS features enabled by default.


This is a summary of points based on the diagram:

• Ingress QoS features such as classification, marking and policing can be configured per port basis.
  
• Input map tables and ingress queueing can be configured globally. These cannot be configured per port basis.
  
• SRR for ingress queue can be configured globally.
  
• Stack ring bandwidth depends on the stack cabling. If the stack is connected at full bandwidth, you receive 32Gbps bandwidth. This bandwidth is shared by all the switches in the stack.
  
• Output map tables and egress queues are configured globally. You can have two sets of queue configurations and you can apply any one of the queue set configurations to a port.
  
• SRR for egress queue can be configured on per port basis. 
  
  
  Default Ingress QoS Configuration
  In summary, the CoS and DSCP values of the frame enter the switch set to 0 by default if the QoS is enabled on the switch.
  
  
  Classification and Marking
  
  The configuration based on the incoming CoS/DSCP value is achieved in three different ways:
  
  • Port based configuration using the mls qos interface based commands
    
  • MQC based configuration using class-map and policy-map
    
  • VLAN based configuration
    
  You can use either one of these three methods. You cannot use more than one method in a port. For example, you have configured the mls qos trust cos command on a port. When you configure the port with the service-policy input <policy-map-name> command, it removes the mls qos trust cos command automatically.
  
  
  Congestion Management and Avoidance
  
  Congestion management and avoidance is a three step process. The steps are queueing, dropping and scheduling. Queueing places the packets into the different software queues based on the QoS labels. The Cisco Catalyst 3750 Switch has two ingress queues. After the traffic is classified and marked with QoS labels, you can assign the traffic into two different queues based on the QoS labels.
  
  Weighted tail drop (WTD) is used to manage the queue lengths and to provide drop precedences for different traffic classifications.
  
  Both the ingress and egress queues are serviced by SRR, which controls the rate at which packets are sent. On the ingress queues, SRR sends packets to the stack ring. SRR can operate in two modes called shaped and shared. For ingress queues, sharing is the default mode, and it is the only mode supported. In shared mode, the queues share the bandwidth among them according to the configured weights. The bandwidth is guaranteed at this level but not limited to it.
  

  Egress QoS Features

  Congestion management and avoidance are the egress QoS features supported by Cisco Catalyst 3750 Switches. Congestion management and avoidance is a three step process. The steps are queueing, dropping and scheduling.
  Queueing places the packets into the different software queues based on the QoS labels. The Cisco Catalyst 3750 Switch has 4 egress queues, 3 threshold per queue. After the traffic is classified and marked with QoS labels, you can assign the traffic into four different queues based on the QoS labels.
  Each queue can be configured with buffer size, reserved threshold, threshold levels, and maximum threshold. Weighted tail drop (WTD) is used to manage the queue lengths and to provide drop precedences for different traffic classifications. Ingress queue parameters are configured globally. Ingress queue parameters are not per port basis. However, egress queue parameters are configured per port basis. Even then the configuration is per port. You cannot configure each port differently. You can configure each port in two different ways. This is called a queue set. You can configure a maximum of two different queue sets in global configuration. Then, you can apply either one of these two sets on the interface.
  Both the ingress and egress queues are serviced by SRR, which controls the rate at which packets are sent. On the ingress queues, SRR sends packets to the stack ring. SRR can operate in two modes called shaped and shared. For ingress queues, sharing is the default mode, and it is the only mode supported. In shared mode, the queues share the bandwidth among them according to the configured weights. The bandwidth is guaranteed at this level but not limited to it. In shaped mode, the egress queues are guaranteed a percentage of the bandwidth, and they are rate-limited to that amount. Shaped traffic does not use more than the allocated bandwidth even if the link is idle. Shaping provides a more even flow of traffic over time and reduces the peaks and valleys of bursty traffic. Queue 1 can be configured as the priority queue.
  
  http://www.cisco.com/en/US/products/hw/switches/ps5023/products_tech_note09186a0080883f9e.shtml