Cisco Systems 3600 manual Frame Relay for Voice over IP Configuration Example

Frame Relay for Voice over IP Configuration Example

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.

Configuring Voice over IP for the Cisco 3600 Series VC-23