Cisco AS5350XM Configuration Manual page 167

Configuring Voice over IP
Congestion Management
Weighted Fair Queuing
You need to avoid congestion on backbone gateways serving high-traffic, high-speed networks. A
weighted-fair-queuing methodology called WRED (weighted random early detection) queues traffic
according to priority values that you set (you set voice traffic to critical, for example), sets different
packet-drop thresholds for each queue, and drops packets in lower-priority queues as necessary so that
higher-priority queues can be adequately served. This ensures that low-bandwidth conversations get
through, even in the presence of other high-bandwidth applications.
For more information and configuration options, see the Configuring Weighted Fair Queuing document,
Tip
available online at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/12cgcr/qos_c/qcpart2/
Low-Latency Queuing
If you need to give voice packets priority but cannot allow them to starve other applications, the
recommended queuing methodology is LLQ (low-latency queuing), used in conjunction with IP RTP
Priority. LLQ directs voice traffic into a priority queue, but allows you to place limits on the amount of
traffic serviced at this and each other priority level before the next-lower priority level is serviced.
For more information and configuration options, see the Low-Latency Queuing document, available
Tip
online at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t7/
IP RTP Priority and Frame Relay IP RTP Priority
IP RTP Priority creates a strict-priority queue for VoIP calls. Only when the priority queue empties does
the gateway process the other queues. The feature becomes active only when congestion exists on the
interface.
Configure IP RTP Priority when you configure dial peers. Set an IP priority level to specify, in the packet
header, that a voice call be accorded class-5 (critical) priority. Other queuing and traffic-management
functions such as RSVP detect this information and provide priority service.
If your voice traffic passes through a Frame Relay network, the same argument holds, but the feature is
called Frame Relay IP RTP Priority (described in the third reference below).
For more information and configuration options, see the following:
Tip
•
ip route-cache
VoIP over PPP Links with Quality of Service (LLQ / IP RTP Priority, LFI, cRTP), available online at
http://www.cisco.com/warp/public/788/voice-qos/voip-mlppp.html
Cisco AS5350XM and Cisco AS5400XM Universal Gateways Software Configuration Guide
Enabling QoS Features for VoIP
17