Configuring Voice over IP
Enabling QoS Features for VoIP
104
Cisco VGD 1T3 Voice Gateway Software Configuration Guide
Tip
For more information and configuration options, see the Configuring Weighted Fair Queuing document,
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.
Tip
For more information and configuration options, see the Low-Latency Queuing document, available
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).
Tip
For more information and configuration options, see the following:
•
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
•
IP RTP Priority, available online at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t5/iprtp.htm
•
Frame Relay IP RTP Priority, available online at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios120/120newft/120t/120t7/
friprtp.htm
Resource Reservation
You can set things up so that your and any other similarly-set-up sending or receiving system can reserve
bandwidth, on a call-by-call basis, along a router path by enabling RSVP (Resource Reservation
Protocol) on all WAN links that transport voice traffic.
Configure RSVP when you configure dial peers. Do not enable RSVP in conjunction with Frame Relay
traffic shaping.