Cisco Voice over IP (CVoice) (Authorized Self-Study Guide) (2nd Edition)

Cisco AutoQoS minimizes the complexity, time, and operating cost of QoS deployment. Cisco AutoQoS incorporates value-added intelligence into Cisco IOS software and Cisco Catalyst Operating System software to provision and manage large-scale QoS deployments. This section focuses on the Cisco IOS implementation of AutoQoS, on both router and Catalyst switch platforms.

AutoQoS Features

To expedite QoS deployment, the user interface must be simplified. Cisco AutoQoS addresses this issue by automating the following five main aspects of QoS deployment while maintaining a tunable solution:

• Application classification Cisco AutoQoS identifies VoIP control and bearer traffic. Cisco AutoQoS uses Cisco Discovery Protocol (CDP) to ensure that the device attached to the LAN is really an IP phone.

• Policy generation Cisco AutoQoS evaluates the network environment and automatically generates an initial policy on a given interface, port, or permanent virtual circuit (PVC).

• Configuration With one command, Cisco AutoQoS configures a port to prioritize voice traffic without affecting other network traffic, while still offering the flexibility to adjust QoS settings for unique network environments. QoS settings are automatically disabled when a Cisco IP phone is relocated or moved.

• Monitoring and reporting Cisco AutoQoS provides visibility into the classes of service deployed via system logging and SNMP traps.

• Consistency Cisco AutoQoS policies are designed to work in harmony with each other across Cisco devices, ensuring consistent end-to-end QoS.

The increased deployment of delay-sensitive applications in networks (for example, voice, video, and other multimedia applications) requires proper QoS configuration to ensure application performance.

Before the availability of Cisco AutoQoS, proper QoS configuration of a network required a deep understanding of various QoS features (that is, queuing, dropping, traffic conditioning, queue depth, drop thresholds, burst parameters, LFI, and RTP). The use of Cisco AutoQoS helps minimize the complexity of configuring a network correctly for QoS by automatically configuring a device with the correct QoS parameters. Cisco AutoQoS automates consistent deployment of QoS features across Cisco routers and switches. It enables various Cisco QoS components based on the network environment and Cisco best-practice recommendations.

Users can subsequently tune parameters generated by Cisco AutoQoS to suit their particular application needs.

Cisco AutoQoS can perform the following functions on WAN interfaces:

• Automatically classifies RTP payload and VoIP control packets (H.323, H.225 Unicast, Skinny Client Control Protocol [SCCP], session initiation protocol [SIP], and Media Gateway Control Protocol [MGCP])

• Builds service policies for VoIP traffic that are based on Cisco's Modular QoS Command-Line Interface (MQC) configuration approach

• Provisions LLQ priority queuing for VoIP bearer and bandwidth guarantees for control traffic

• Enables WAN traffic shaping that adheres to Cisco best practices, where required

• Enables link efficiency mechanisms, such as LFI and RTP header compression (cRTP), where required

• Provides SNMP and syslog alerts for VoIP packet drops

Cisco AutoQoS can perform the following functions on LAN interfaces:

• Enforce the trust boundary on Cisco Catalyst switch access ports, uplinks, and downlinks

• Enable Cisco Catalyst strict priority queuing (also known as expedite queuing) with weighted round-robin (WRR) scheduling for voice and data traffic, where appropriate

• Configure queue admission criteria (that is, maps Layer 2 CoS priority markings in incoming packets to the appropriate queues)

• Modify queue sizes and weights where required

Note

Cisco AutoQoS is available in the following Cisco IOS software releasesCisco IOS Software Release 12.1E or later for the Cisco Catalyst 2950 and 3550 Series switches; Cisco IOS Software Release 12.2T or later for the Cisco 2600, 2600XM, 3600, 3700, and 7200 Series routers; Cisco IOS Software Release 12.1E or later for the Cisco Catalyst 4500 Series switches; and Cisco Catalyst Operating System 7.5.1 or later for the Cisco Catalyst 6500 Series switches. For current information concerning AutoQoS platform support, check Cisco's Feature Navigator at http://www.cisco.com/go/fn.

Configuring AutoQoS on a Router

On a router platform, the following command enables AutoQoS from either interface configuration mode or from DLCI configuration mode (for a Frame Relay circuit):

Router(config-if)#auto qos voip [trust] [fr-atm]

The trust option indicates that AutoQos should classify voice traffic based on Layer 3 Differentiated Services Code Point (DSCP) priority markings, instead of using Network-Based Application Recognition (NBAR). The fr-atm option enables the AutoQoS feature for Frame Relay-to-ATM links and is issued from DLCI configuration mode.

Before enabling AutoQoS on a router interface, consider the following prerequisites:

• Cisco Express Forwarding (CEF) must be enabled, because AutoQoS uses NBAR, which requires the CEF feature.

• A QoS policy must not be currently attached to the interface.

• The correct bandwidth should be configured on the interface, using the bandwidth command.

• An IP address must be configured on an interface if its speed is less than 768 kbps.

• The interface must not be administratively shut down.

Note that the interface's bandwidth determines which AutoQoS features are enabled. If an interface's bandwidth is less than 768 kbps, it is considered a low-speed interface. On a low-speed interface, AutoQoS configures Multilink PPP (MLP), which requires an IP address on the physical interface. AutoQoS takes the IP address from the physical interface and uses it for the virtual multilink interface that it creates.

To verify that AutoQoS is configured for a router interface, use the following command:

Router#show auto qos [interface interface-identifier]

To illustrate some of the configuration changes that AutoQoS can perform, consider the configuration of a serial interface shown in Example 7-1, without AutoQoS enabled.

Example 7-1. Router Configuration Without AutoQoS

Example 7-2 illustrates the configuration changes after entering the auto qos voip command for interface Serial 0/0.

Example 7-2. Router Configuration with AutoQoS

The bandwidth configured for interface Serial 0/0 was set to 128 kbps. Therefore, AutoQoS determined that certain link efficiency mechanisms (for example, Multilink PPP, RTP Header Compression, and TCP Header Compression) were appropriate. AutoQoS, therefore, automatically configured these link efficiency mechanisms in addition to multiple other QoS mechanisms, including classification, marking, LLQ, and Remote Monitoring (RMON) traps to alert administrators if packet drops are excessive.

AutoQoS for Enterprise

Introduced in Cisco IOS Software Release 12.3(7)T, AutoQoS for Enterprise extends the capabilities of AutoQoS on a Cisco router platform. Specifically, AutoQoS for Enterprise allows a router to recognize multiple protocols traversing an interface and recommends a customized policy, based on learned traffic patterns.

To configure a router's interface to begin learning traffic patterns, enter the following command in interface configuration mode:

Router(config-if)#auto discovery qos

After entering the previous command, wait for a period of time for the router to learn the traffic patterns crossing the interface. The ability to dynamically learn these patterns is made possible by the Cisco IOS software's NBAR feature.

After waiting a period of time (for example, 30 minutes to an hour in a time period representative of peak network usage) during which the router is learning the traffic patterns of the network, enter the following command to view the router's findings and to see the recommended policy for the interface:

Router#show auto discovery qos

Example 7-3 offers an example of the output received after issuing the show auto discovery qos command.

Example 7-3. Output from the show auto discovery qos Command

If you find the suggested policy acceptable and wish to apply the dynamically created policy, go into interface configuration mode for the monitored interface and enter the following command:

Router(config-if)#auto qos

This auto qos command applies the recommended policy to the router.

Configuring AutoQoS on a Catalyst Switch

The QoS mechanisms on a Catalyst switch differ from those QoS mechanisms found on a router. For example, while a router uses LLQ as a priority queuing strategy, a Catalyst switch might use weighted round-robin (WRR) as a priority queuing strategy. Fortunately, the AutoQoS feature available on some Catalyst switch models (for example, the Cisco Catalyst 2950(EI) and 3550 Series) apply voice-specific QoS features globally to a Catalyst switch and also at the port level.

To configure AutoQoS on supported Catalyst switch platforms (running the Native IOS), issue the following command from interface configuration mode:

Switch(config-if)#auto qos voip [trust | cisco-phone]

If the trust option is used in the previous command, the Catalyst switch makes queuing decisions based on Layer 2 Class of Service (CoS) markings. However, if the cisco-phone option is used, the Catalyst switch makes queuing decisions based on CoS markings originating from a Cisco IP phone. The switch detects the presence of a Cisco IP phone via the CDP.

To illustrate the configuration changes made by a Catalyst switch's AutoQoS feature, consider Example 7-4, which shows the initial configuration of interface Gigabit 0/1 on a Catalyst 3550 switch.

Example 7-4. Catalyst Switch Configuration Without AutoQoS

Example 7-5 illustrates the configuration changes after entering the auto qos voip cisco-phone command for interface Gigabit 0/1.

Example 7-5. Catalyst Configuration with AutoQoS

Example 7-5 demonstrates that the AutoQoS feature configured the Catalyst switch to enable QoS globally (with the mls qos command) and remark Layer 2 CoS markings to Layer 3 Differentiated Services Code Point (DSCP) markings (with the mls qos map cos-dscp command). Also, AutoQoS configured WRR for interface Gigabit 0/1 and placed traffic with a CoS value of 5 (that is, voice traffic) in a priority queue, which is emptied ahead of other queues.