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Frame Relay for Voice over IP Configuration Example
Configuring Voice over IP for the Cisco 3600 Series VC-23
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. This is
accomplished through generic traffic shaping.
— Use IP Precedence to prioritize voice traffic.
— Use compressed RTP to minimize voice packet header size.
• Traffic shaping—Use adaptive traffic shaping to throttle back the output rate based on the 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.
— 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 Bc to 4000
to obtain an inter-packet wait of 125 ms.
Note We recommend FRF.12 fragmentation setup rules for Voice over IP connections over Frame
Relay. FRF.12 was implemented in the Cisco IOS Release 12.0(4)T. For more information, refer to
the Cisco IOS Release 12.0(4)T “Voice over Frame Relay using FRF.11 and FRF.12” feature
module.
Frame Relay for Voice over IP Configuration Example
For Frame Relay, it is customary to configure a main interface and several subinterfaces, 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
ip 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 Serial0/0.1 point-to-point
ip mtu 300
ip address 40.0.0.7 255.0.0.0
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 has been configured as follows:
• MTU size of IP packets 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.