Specifications

Configuration Tasks
12
Cisco IOS Release 12.0(3)T
In general, weighted fair queueing is used in conjunction with Multilink PPP with interleaving and
RSVP or IP Precedence to ensure that voice packet delivery. Use weighted fair queueing with
Multilink PPP to define how data will be managed; use RSVP or IP Precedence to give priority to
voice packets. For more information about weighted fair queueing, refer to the Cisco IOS Release
12.0 Quality of Service Solutions Configuration Guide.
Configure Frame Relay for Voice Over IP
You need to consider certain factors when configuring Voice over IP for it to run smoothly over
Frame Relay. A public Frame Relay cloud provides no guarantees for QoS. For real-time traffic to
be transmitted in a timely manner, the data rate must not exceed the CIR or there is the possibility
that packets will be dropped. In addition, Frame Relay traffic shaping and RSVP are mutually
exclusive, which is particularly important to remember if multiple DLCIs are carried on a single
interface.
For Frame Relay links with slow output rates (less than or equal to 64 kbps), where data and voice
are being transmitted over the same PVC, Cisco recommends the following solutions:
• Separate DLCIs for voice and data—By providing a separate subinterface for voice and data, you
can use the appropriate QoS tool per line. For example, each DLCI would use 32 kbps of a
64-kbps line.
— Apply adaptive traffic shaping to both DLCIs.
— Use RSVP or IP Precedence to prioritize voice traffic.
— Use compressed RTP to minimize voice packet size.
— Use weighted fair queueing to manage voice traffic.
• Lower MTU size—Voice packets are generally small. If you lower the MTU size (for example,
to 300 bytes), large data packets can be broken up into smaller data packets that can more easily
be interwoven with voice packets.
Note Some applications do not support a smaller MTU size. If you decide to lower MTU size,
use the ip mtu command; this command affects only IP traffic.
Note Lowering the MTU size affects data throughput speed.
• CIR equal to line rate—Make sure that the data rate does not exceed the CIR by using generic
traffic shaping.
— Use compressed RTP to minimize voice packet header size.
• Traffic shaping—Use adaptive traffic shaping to throttle back the output rate based on the
backward explicit congestion notification (BECN) bit. If the feedback from the switch is ignored,
packets (both data and voice) might be discarded. Because the Frame Relay switch does not
distinguish between voice and data packets, voice packets could be discarded, which would result
in a deterioration of voice quality.
— Use compressed RTP, reduced MTU size, and adaptive traffic shaping based on BECN to
hold data rate to CIR.
— Use generic traffic shaping to obtain a low interpacket wait time. For example, set the Bc
parameter to 4000 to obtain an interpacket wait of 125 milliseconds.