Cisco AutoQoS VoIP from a different
subinterface. For low-speed Frame Relay DLCIs
configured for use on Frame Relay-to-ATM Interworking, MLP over
Frame Relay is configured automatically. The subinterface must
have an IP address. When MLP over Frame Relay is
configured, this IP address is removed and put on the MLP
bundle. You must also configure Cisco AutoQoS on the ATM side
of the network. For low-speed Frame Relay DLCIs with
Frame Relay-to-ATM Interworking, you cannot configure Cisco
AutoQoS if a virtual template is already configured for the
DLCI. ATM PVC Restrictions
Cisco AutoQoS
has the following restrictions in ATM environment: -
For a low-speed ATM PVC, you cannot configure Cisco AutoQoS if
a virtual template is already configured for the ATM PVC.
- For low-speed ATM PVCs, MLP over ATM is configured
automatically. The subinterface must have an IP address.
- When MLP over ATM is configured, this IP address is
removed and put on the MLP bundle.
Content
5.1 Introducing Cisco AutoQoS 5.1.5
Router Design Considerations When you are configuring
Cisco AutoQoS, take the design considerations for Cisco router
platforms summarized in Figure into account as follows:
- General QoS requirements: Recommended methods and
values are configured to meet the QoS requirements for
real-time traffic. Cisco AutoQoS takes the interface type and
bandwidth into consideration when implementing these QoS
features:
- Low latency queuing (LLQ) priority
queue: LLQ (specifically, the priority queue) is applied to
the voice packets to meet the latency requirements. LLQ gives a
priority to voice Real-Time Transport Protocol (RTP) packets
over other data traffic types when they share an output link
with voice.
- Compressed Real-Time Transport
Protocol (cRTP): With cRTP, the 40-byte IP header of the
voice packet is reduced to 2 or 4 bytes, reducing voice
bandwidth requirements. This mechanism is used on low-speed
serial links to improve link efficiency and decrease the RTP
packet overhead caused by extensive voice packet headers. You
must apply cRTP at both ends of a network link.
-
Link fragmentation and interleaving (LFI): LFI reduces
jitter for voice packets by preventing voice packets from being
delayed by large data packets in a queue when real-time voice
and bursty data traffic share the same low-speed output link.
You must apply LFI at both ends of a network link.
- Bandwidth implications: The bandwidth of the
serial interface determines the speed of the link. The speed of
the link, in turn, determines the configurations generated by
Cisco AutoQoS.
- Note that changing the bandwidth during
configuration of Cisco AutoQoS is not recommended.
-
Cisco AutoQoS uses the bandwidth allocated at the time that the
feature is configured. Cisco AutoQoS does not respond to
changes made to bandwidth after the feature is
configured.
- Fragmentation for Frame
Relay Networks: For Frame Relay networks, fragmentation is
configured based on G.729 using a delay of 10 ms and a minimum
fragment size of 60 bytes. This configuration ensures that the
VoIP packets are not fragmented. However, when you use the
G.711 coder-decoder (codec) on low-speed links, the fragment
size configured by Cisco AutoQoS could be smaller than the size
of the G.711 VoIP packet. To solve this potential problem,
choose one of these options:
- Change the fragment size
to the required value.
- Replace the G.711 codec with a
codec more suitable for low-bandwidth links; for instance,
G.729.
For example, if you configure Cisco AutoQoS on a
Frame Relay DLCI with 128 kbps, the fragment size configured by
Cisco AutoQoS for the Enterprise will be 160 bytes. The size of
the G.711 VoIP packet will be 160 bytes, plus the bytes in the
packet headers for the layers. The workaround is to either
change the fragment size from 160 bytes to 220 bytes or use
G.729 or another codec that produces packets smaller than the
fragment size.
Content 5.1
Introducing Cisco AutoQoS 5.1.6 Router
Prerequisites Before configuring Cisco AutoQoS, you must
meet the prerequisites listed in Figure . Details of these
prerequisites are as follows: - You must ensure the
interface has no QoS policies (service policies) attached. You
cannot configure Cisco AutoQoS if the interface has a QoS
policy attached.
- You must enable Cisco Express
Forwarding (CEF). Cisco AutoQoS uses NBAR to identify various
applications and traffic types, and CEF is a prerequisite for
NBAR.
- Cisco AutoQoS classifies links as either low
speed or high speed depending on the link bandwidth. Remember
that on a serial interface, if you do not specify the
bandwidth, the default bandwidth will be 1.544 Mbps. Therefore,
it is important that the correct bandwidth be specified on the
interface or subinterface where Cisco AutoQoS is to be
enabled:
- For all interfaces or subinterfaces, use the
bandwidth command to configure the bandwidth properly.
Base the amount of bandwidth on the link speed of the
interface.
- If the interface or subinterface has a link
speed of 768 kbps or lower, you must configure an IP address on
the interface or subinterface using the ip address
command. By default, Cisco AutoQoS enables MLP and copies the
configured IP address to the multilink bundle interface.
In addition to the Cisco AutoQoS
prerequisites, there are other recommendations and requirements
for configuring Cisco AutoQoS. Be aware that these may change
with Cisco IOS software releases. Verify the prerequisites
before implementing Cisco AutoQoS in any environment:
- Cisco AutoQoS is supported only on these interfaces and
PVCs:
- ATM PVCs
- Serial interfaces with PPP or
HDLC
- Frame Relay DLCIs (point-to-point subinterfaces
only, because Cisco AutoQoS does not support Frame Relay
multipoint interfaces)
- A configuration
template generated by configuring Cisco AutoQoS on an interface
or PVC can be tuned manually (via CLI configuration) if
desired.
- To include SNMP traps (monitored events), you
must enable SNMP support on the router. Cisco AutoQoS SNMP
traps are delivered only when you use an SNMP server in
conjunction with Cisco AutoQoS and the router knows how to
reach the SNMP server.
- The SNMP community string
“AutoQoS” should have write permission.
- If you reload
the device with the saved configuration after configuring Cisco
AutoQoS and saving the configuration to NVRAM, Remote
Monitoring (RMON) threshold commands may generate some warning
messages. You can ignore these warning messages. (To avoid
further warning messages, save the configuration to NVRAM again
without making any changes to the QoS configuration.)
- By default, Cisco Routers reserve up to 75 percent of the
interface bandwidth for user-defined classes. The remaining
bandwidth is kept for the default class. However, the entire
remaining bandwidth is not guaranteed to the default class. The
default class and excess traffic from other bandwidth classes
share this bandwidth proportionately.
Content
5.1 Introducing Cisco AutoQoS
5.1.7 Deploying Cisco AutoQoS for the Enterprise on
Routers: A Two-Step Approach Cisco AutoQoS for the
Enterprise consists of two configuration phases as shown in
Figure : - Auto Discovery (data collection)
-
Cisco AutoQoS template generation and installation
The Auto Discovery phase uses NBAR-based Protocol Discovery to
detect the applications on the network and perform statistical
analysis on the network traffic. The data collected should be a
representative sampling of the volume and type of voice, video,
and data on your network. Therefore, the amount of time devoted
to data collection varies from network to network. Run the Auto
Discovery phase for as long as necessary. The length of time