Cisco Catalyst Switch QoS Features 721

Table 10-16 Catalyst 4500/4000 Supervisor III or IV QoS Features (Continued)

Cisco Catalyst 3550 QoS Features

The Catalyst 3550 family of switches supports enhanced QoS features that can be used in the access and distribution layers. The transmit queue on the 3550 series of switches includes one priority queue, which resides in Queue 4, and three standard queues with a single threshold (1p3q1t). Scheduling is done on a WRR basis, where each queue is given a relative weight, while the priority queue is serviced exhaustively. The default is WRR only. If priority scheduling is required, it must be explicitly enabled. Admission to the queues is based on IP DSCP, 802.1p CoS, or port-priority CoS. Table 10-17 describes the default queue admission criteria.

Table 10-17 Catalyst 3550 Default Queue Admission

Similar to the Catalyst 4500/4000 Supervisor III or IV, all ports on the Catalyst 3550 are in the untrusted state after QoS has been enabled. Trust is enabled on a per-port basis. The Catalyst 3550 also has the capability to extend trust to the ASICs in the IP Phones, allowing the switch to trust the IP Phones without having to trust the attached PC.

The Catalyst 3550 also has the capability to classify and mark traffic on ingress to the network using standard and extended IOS ACLs. The Catalyst 3550’s capability to identify traffic flows at Layer 3 and Layer 4 using ACLs makes it very powerful as an access layer switch.

722 Chapter 10: LAN QoS

Table 10-18 lists the available QoS features of a Catalyst 3550.

Table 10-18 Catalyst 3550 QoS Features

Cisco Catalyst 3524 QoS Features

The available inline power, ease of configuration, and priority queuing capabilities have made the Catalyst 3524 switch a popular Layer 2 switch for IP telephony deployments. Although the 3524 has been replaced with the 3550-24PWR for future deployments, the number of deployed 3524 switches warrants a brief discussion of its QoS features.

QoS is enabled by default on the Catalyst 3524 switch. The transmit queue on the Catalyst 3524 series of switches includes one priority queue and one standard queue (1p1q). By default, voice bearer traffic (typically marked as CoS 5) is mapped to the strict-priority queue, which resides in Queue 2. Table 10-19 describes the default queue admission criteria.

Table 10-19 Catalyst 3524 Default Queue Admission

The 3524 trusts the CoS values received on each port by default and places the traffic in the appropriate queue based on the CoS value received. However, the Catalyst 3524 does offer the capability to extend the trust to the ASICs on IP Phones, thereby trusting the IP Phones without having to trust the attached PC.

QoS Configurations on Catalyst Switches 723

Table 10-20 lists the available QoS features of a Catalyst 3524.

Table 10-20 Catalyst 3524 QoS Features

QoS Configurations on Catalyst Switches

Several factors determine the configuration of a Cisco Catalyst switch. It is important to fully understand the QoS requirements that are needed to support the applications on your network before adding real-time applications such as voice and video. Ask yourself the following questions:

•What are the QoS requirements for the LAN running the current applications?

•What are the QoS requirements of the LAN if a real-time application is added?

•How many queues are present on the switches that comprise the LAN?

•Can the access layer switches support the QoS mechanisms that are needed to support the addition of real-time applications?

•Can the distribution layer switches support the QoS mechanisms that are needed to support the addition of real-time applications?

•Do the LAN switches support a priority queue?

•Can the LAN switches classify traffic on Layer 3 markings?

•Will the LAN need to be upgraded to handle the addition of real-time applications?

•Do the LAN switches use Catalyst OS or IOS operating system?

After you understand the QoS LAN requirements, you can begin to examine the configuration steps necessary to meet the requirements. The following configuration sections describe QoS configurations on Catalyst OS and IOS switches in both the distribution and access layer of your network. These configurations are based on Cisco QoS Solution Reference Network Design (SRND), which you can find on the Cisco website at the following link:

www.cisco.com/warp/customer/771/srnd/qos_srnd.pdf

QoS Configurations on Catalyst Switches 725

Configuring Auxiliary VLANs for a Catalyst OS Switch

The first step in configuring QoS for the access layer is separating the voice traffic from the data traffic. Cisco IP Phones have the capability to use 802.1Q trunks to accomplish this task. The IP Phone can tag the voice traffic with a VLAN identifier, while leaving the data traffic in the native, or untagged, VLAN. The switch must be configured to participate in the 802.1Q trunk from the IP Phone. This is achieved with the set port auxiliaryvlan command. Example 10-7 shows the auxiliary VLAN of 110 being set for ports 2/1 through 2/48.

Example 10-7 Creating Auxiliary VLANs

CatOS> (enable) set port auxiliaryvlan 2/1-48 110

In this example, switch ports 2/1 through 2/48 have been configured to participate in the 802.1Q trunk from the phones using VLAN 110 for the tagged VLAN that will carry the voice traffic.

Configuring QoS for the Catalyst OS Switch

QoS must be enabled globally within the switch to use the multiple queues desired. This is achieved with the set qos enable command, as shown in Example 10-8.

Example 10-8 Enabling QoS

CatOS> (enable) set qos enable

QoS is enabled.

CatOS> (enable)

In this example, QoS has been enabled on the Catalyst OS switch.

After QoS has been enabled, it can be configured on a specific port or on a per-VLAN basis. Configuring QoS on a port applies QoS to only the specified port, whereas configuring QoS on a VLAN applies the QoS configuration to all ports that reside in that VLAN.

The set port qos command is used to associate QoS configuration with a specific port or a VLAN. Example 10-9 assigns VLAN QoS configuration to ports 2/1 through 2/48 based on VLAN membership.

Example 10-9 Associating QoS with a VLAN

CatOS> (enable) set port qos 2/1-48 vlan-based

Hardware programming in progress...

QoS interface is set to vlan-based for ports 2/1-48.

CatOS> (enable)