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

interface Serial0/0
 bandwidth 128
 ip address 10.1.1.1 255.255.255.0
 encapsulation ppp

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

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class-map match-any AutoQoS-VoIP-Remark
class-map match-any AutoQoS-VoIP-Remark
  match ip dscp ef
  match ip dscp cs3
  match ip dscp af31
class-map match-any AutoQoS-VoIP-Control-UnTrust
  match access-group name AutoQoS-VoIP-Control
class-map match-any AutoQoS-VoIP-RTP-UnTrust
  match protocol rtp audio
  match access-group name AutoQoS-VoIP-RTCP
!
policy-map AutoQoS-Policy-UnTrust
  class AutoQoS-VoIP-RTP-UnTrust
   priority percent 70
   set dscp ef
  class AutoQoS-VoIP-Control-UnTrust
   bandwidth percent 5
   set dscp af31
  class AutoQoS-VoIP-Remark
   set dscp default
  class class-default
   fair-queue
!
interface Multilink2001100114
 bandwidth 128
 ip address 10.1.1.1 255.255.255.0
 service-policy output AutoQoS-Policy-UnTrust
 ip tcp header-compression iphc-format
 ppp multilink
 ppp multilink fragment delay 10
 ppp multilink interleave
 ppp multilink group 2001100114
 ip rtp header-compression iphc-format
!
interface Serial0/0
 bandwidth 128
 no ip address
 encapsulation ppp
 auto qos voip
 clockrate 128000
 no fair-queue
 ppp multilink
 ppp multilink group 2001100114
!
ip access-list extended AutoQoS-VoIP-Control
 permit tcp any any eq 1720
 permit tcp any any range 11000 11999
 permit udp any any eq 2427
 permit tcp any any eq 2428
 permit tcp any any range 2000 2002
 permit udp any any eq 1719
 permit udp any any eq 5060
ip access-list extended AutoQoS-VoIP-RTCP
 permit udp any any range 16384 32767
!
rmon event 33333 log trap AutoQoS description "AutoQoS SNMP traps for Voice Drops" owner
 AutoQoS
rmon alarm 33333 cbQosCMDropBitRate.1081.1083 30 absolute rising-threshold 1 33333
 falling-threshold 0 owner 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