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Exploring QoS in Catalyst

  • Sample Chapter is provided courtesy of Cisco Press.
  • Date: Jul 4, 2003.

Chapter Description

Discover the workings of the the various platform QoS features available across the Cisco Catalyst product family. You will gain full understanding of each Catalyst platform's supported QoS features and be able to configure various Catalyst families of switches.

QoS Support on the Catalyst 2900XL and 3500XL

Specific models of the Catalyst 2900XL and 3500XL support QoS classification and congestion management. Ingress packet CoS values and configured port priorities exclusively determine classification of ingress frames for placement into either a low-priority or high-priority global transmit queue. The two global transmit queues with priority scheduling create the congestion management mechanism. The following sections discuss these QoS features with detailed overviews, configuration guidelines, and examples.

Catalyst 2900XL Product Family Delineation

QoS support on the Catalyst 2900XL and 3500XL platforms is software and model dependent. For the 2900XL, the original-edition models do not support QoS features, including the uplink modules for the Catalyst 2916M. All standard- and enterprise-edition models do support QoS features. Table 3-6 indicates which 2900XL models support QoS features. All models of the 3500XL support QoS features. In addition, the Catalyst 3524XL-PWR-XL and the 3548XL support CoS reclassification. This QoS feature is not available on other 3500XL platforms.

Table 3-6 QoS Support by Model of 2900XL

Catalyst 2900XL/3500XL Model


QoS Support


8-port 10/100BASE-TX switch



12-port 10/100BASE-TX switch



12-port 100BASE-FX switch



16-port 10/100BASE-TX switch + 2 uplink slots



24-port 10/100BASE-TX switch



22-port 10/100BASE-TX + 2-port 100BASE-FX switch



24-port 10/100BASE-TX switch



22-port 10/100BASE-TX switch + 2-port 100BASE-FX switch



24-port 10/100BASE-TX switch + 2 uplink slots



24-port 10/100BASE-TX switch + 2 uplink slots (DC power)


Catalyst 2900XL and 3500XL QoS Architectural Overview

The Catalyst 2900XL and 3500XL switches are limited to QoS features that suit access layer switches. These features include classification, marking, and congestion management via the use of output scheduling. Because of these features, the 2900XL and 3500XL fit well into an end-to-end QoS design with core switches. Figure 3-2 shows a sample network deploying access layer QoS features with comprehensive QoS features in the core.

Figure 02Figure 3-2 Network Topology Using Catalyst 3524XLs

Figure 3-3 shows the basic QoS architecture model for the Catalyst 2900XL and 3500XL discussed in the following sections.

Figure 03Figure 3-3 Basic QoS Architecture for the Catalyst 2900XLs and 3500XLs Switches

Software Requirements

For QoS feature support, the 2900XL and 3500XL require Cisco IOS Software Release 12.0(5)XP or higher. The 3524-PWR-XL and the 3548XL require 12.0(5)XU or higher for the reclassification of class of CoS values in frames.

Input Scheduling

The Catalyst 2900XL and 3500XL do not perform input scheduling as ingress packets are immediately copied to a global, shared memory buffer. As long as the packet-forwarding rate of the switch is not exceeded, input congestion is not critical to implementing QoS. The packet-forwarding rates of the Catalyst 2900XL and the Catalyst 3500XL are 1.6 Gbps and 5.0 Gbps, respectively.


The Catalyst 2900XL and 3500XL switches both support classification of untagged frames. Two models of the Catalyst 3500XL switch, the Catalyst 3524-PWR-XL and the Catalyst 3548XL switches, support marking of ingress tagged frames. Classification and marking is configurable only on a per-port basis, and each port may be configured with a unique CoS value to be classified.

To configure a Catalyst 2900XL and 3500XL for classification or marking of frames, use the following interface command:

switchport priority {default default-priority-id | extend {cos value | none |
trust} | override}
  • The default-priority-id parameter is the CoS value to be assigned to untagged ingress frames.

  • The extend option is to configure the 802.1p trust configuration of the connected appliance on the P1 port of the IP Phone. For example, a Cisco IP Phone can be configured to trust or reclassify frames received on its P1 port.

  • The override option is used to mark tagged frames with the default-priority-id. Only the Catalyst 3524-PWR-XL and the Catalyst 3548XL switches support this marking feature.

Example 3-1 shows a Catalyst 3548XL switch port configured to classify untagged frames with a CoS value of 2.

Example 3-1 Catalyst 3548XL Switch Port Configured to Classify Untagged Frames

Switch#show running-config
Building configuration...

Current configuration:
(text deleted)
interface FastEthernet0/48
 switchport priority default 2
 spanning-tree portfast
 (text deleted)

Example 3-2 shows a Catalyst 3524-PWR-XL switch port configured for a voice VLAN. In Cisco IOS Software Release 12.0.x, voice VLAN ports must be configured as trunk ports.

Example 3-2 Catalyst 3548XL Switch Port for Voice VLANs

Switch#show running-config
Building configuration...

Current configuration:
(text deleted)
interface FastEthernet0/24
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport mode trunk
 switchport voice vlan 70
 spanning-tree portfast
(text deleted)


The Catalyst 2900XL and 3500XL software configuration for voice VLANs differs from the Catalyst switches that run Cisco IOS Software Release 12.1, such as the Catalyst 4000 Supervisor III and IV. Cisco IOS Software Release 12.1 does not require voice VLAN ports to be configured as trunks.

Congestion Management

The 2900XL and 3500XL switches use a shared memory buffer system because each individual port does not have its own output queue. This shared memory buffer is divided into two global transmit queues. Each ingress packet is placed into one of two global transmit queues based on CoS value for tagged frames and CoS classification for untagged frames. One of the transmit queues is designated for packets with a CoS value of 0 to 3, and the other transmit queue is reserved for packets with a CoS value of 4 to 7. The queues use a 100-percent threshold value. These queues are not configurable for different CoS values or thresholds. This queue scheme creates a logical high-priority and low-priority queuing mechanism. Priority scheduling is applied such that the high-priority queue is consistently serviced before the low-priority queue. The use of two global transmit queues based on CoS value is default behavior and cannot be altered. As a result, no global configuration is required to enable QoS output scheduling.


Untagged packets that are classified with a CoS value transmitted on trunk ports are appropriately tagged with an 802.1q header with the respective CoS. For packets transmitted on nontrunk ports, the untagged classification only determines which queue the frame is placed in for egress transmission.

Case Study: Classification and Output Scheduling on Cisco Catalyst 3500XL Switches

To demonstrate classification and output scheduling on the Catalyst 3500XL series, a Catalyst 3524-PWR-XL was set up with two Cisco 7960 IP Phones, a Call Manager, and a traffic generator connected to three Fast Ethernet ports and a Gigabit Ethernet port, respectively. Figure 3-4 shows this topology. Two trials were conducted taking voice quality statistical measurements from each IP Phone based on a 1-minute, G7.11 voice call between IP Phone 1 and 2. To create traffic congestion, the traffic generator attached to Gigabit Ethernet port was sending multicast at line rate with a CoS value of 0. The multicast traffic was flooded to all ports, including the Fast Ethernet IP Phones, causing output congestion.

Figure 04Figure 3-4 Catalyst 3500XL Case Study Network Diagram

The Catalyst 3524XL switch was running software version 12.0(5)WC5 for the trial. The configuration only included voice VLANs on the Cisco IP Phone ports. The remaining port configuration of the switch was default. Example 3-3 shows the relevant configuration.

Example 3-3 Catalyst 3548XL Switch Port Configuration for Case Study

Switch#show running-config
Building configuration...

Current configuration:
(text deleted)

interface FastEthernet0/1
 switchport access vlan 70
interface FastEthernet0/2
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport mode trunk
 switchport voice vlan 70
 spanning-tree portfast
interface FastEthernet0/3
 switchport trunk encapsulation dot1q
 switchport trunk native vlan 2
 switchport mode trunk
 switchport voice vlan 70
 spanning-tree portfast
(text deleted)

interface GigabitEthernet0/1
 switchport access vlan 70
(text deleted)

The variant in the two trials was the CoS value placed on the telephony frames between the IP Phones. With a CoS value of 0, the telephony stream was treated with a low priority (the same priority as the multicast traffic). With a CoS value of 5, the telephony stream was treated with a high priority. Table 3-7 summarizes the number of frames transmitted and lost as well as jitter from each trial.

Table 3-7 QoS Trial Results on Catalyst 3524-PWR-XL


Total Frames Transmitted (Phone 1/2)

No. of Receive Lost Frames (Phone 1/2)

Maximum Recorded Jitter (Phone1/2)

CoS = 0 on voice stream



51/49 ms

CoS = 5 on voice stream



51/49 ms

As indicated in Table 3-7, the Catalyst 3524XL did not drop a single frame due to output congestion on the IP Phone ports for packets with a CoS value of 5. Similar results are achievable with multiple Cisco IP Phones in a campus network using the Catalyst 2900XL and 3500XL. The jitter did not vary between the trials because all Catalyst switches drop frames under congestion and only buffer a few frames. The maximum recorded jitter is around 50 ms, which is above the recommended 30 ms for Voice over IP (VoIP). Only the first few frames of the IP flow recorded jitter near 50 ms.


The Catalyst 2900XL and 3500XL suit basic QoS needs for an access layer switch. If additional features such as policing and classification based on DSCP are required, network designers should consider the Catalyst 2950 and 3550 switches for use as an access layer switch. You can summarize QoS feature support on the Catalyst 2900XL and 3500XL switches as follows:

  • No support for input scheduling.

  • Classification based on CoS only; no support for classification based on IP precedence or DSCP.

  • Two global queues for high-priority and low-priority traffic.

  • No configurable CoS mapping to queues or queue threshold.

  • Ports are trusted by default.

  • Untagged frames are mapped to high-priority or low-priority queues based on configured classification CoS value.

  • The Catalyst 3548XL and 3524-PWR-XL support reclassification of tagged frames.

4. QoS Support on the Catalyst 4000 CatOS Family of Switches | Next Section Previous Section

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