Specifications
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Cisco 1751 Router Software Configuration Guide
OL-1070-01
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Use RSVP or IP precedence to prioritize voice traffic.
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Use compressed RTP to minimize voice packet header size.
• Traffic shaping—Use adaptive traffic shaping to slow the output rate based on the backward explicit
congestion notification (BECN). 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.
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Use RSVP, compressed RTP, reduced MTU size, and adaptive traffic shaping based on BECN
to hold data rate to CIR.
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Use generic traffic shaping to obtain a low interpacket wait time. For example, set committed
burst (Bc) to 4000 to obtain an interpacket wait of 125 milliseconds.
In Cisco IOS Release 12.1T, Frame Relay traffic shaping is not compatible with RSVP. We suggest one
of the following workarounds:
• Provision the Frame Relay PVC to have the CIR equal to the port speed.
• Use generic traffic shaping with RSVP.
Frame Relay for VoIP Configuration Example
For Frame Relay, it is customary to configure a main interface and several subinterfaces with one
subinterface per PVC. The following example configures a Frame Relay main interface and a
subinterface so that voice and data traffic can be successfully transported:
interface Serial0/0
mtu 300
no ip address
encapsulation frame-relay
no ip route-cache
no ip mroute-cache
fair-queue 64 256 1000
frame-relay ip rtp header-compression
interface Serial1/0 point-to-point
mtu 300
ip address 40.0.0.7 255.0.0.0
ip rsvp bandwidth 48 48
no ip route-cache
no ip mroute-cache
bandwidth 64
traffic-shape rate 32000 4000 4000
frame-relay interface-dlci 16
frame-relay ip rtp header-compression
In this configuration example, the main interface is configured as follows:
• MTU size is 300 bytes.
• No IP address is associated with this serial interface. The IP address must be assigned for the
subinterface.
• Encapsulation method is Frame Relay.
• Fair-queuing is enabled.
• IP RTP header compression is enabled.
The subinterface is configured as follows: