Configuring FRF12 Frame Relay Fragmentation

This topic describes the Cisco IOS commands that are required to configure FRF. 12.

This topic describes the Cisco IOS commands that are required to configure FRF. 12.

FRF. 12 fragmentation is configured within the Frame Relay map class. The frame-relay fragment command sets the maximum fragment size in bytes. On an interface, the frame-relay class command applies the map class to the interface, subinterface, or a DLCI.

FRF. 12 requires FRTS to be enabled.

The figure shows a configuration example where FRF. 12 fragmentation is applied to a data Frame Relay circuit configured on the serial 0/0.1 subinterface.

The maximum fragment size is set to 80 bytes. This would be used in a VoIP over Frame Relay environment.

FRTS is enabled on the interface and the FRTS token bucket parameters are configured within the Frame Relay map class. In this figure, FRTS is enabled with a Committed Information Rate (CIR) of 64 kbps, a committed burst (Bc) (normal burst size) of 640 bits, and uses weighted fair queuing (WFQ) as the shaping queue.

Note The purpose of fragmentation is to decrease serialization delays so that large data frames cannot block small voice packets for excessive periods of time. Both voice and data packets are subject to the shaping rate. FRTS token bucket operation is based on burst of credits in bits. If a large data packet gets through the shaper and uses up all the credits, the voice packets will be delayed until more credit becomes available. But if only a small fragment of a data packet can get through the shaper, the voice packet will be delayed for less time. So with FRTS and FRF.12, the bottleneck is at the shaper, not at the port. Therefore, the fragment size should be determined based on the CIR and not the physical interface clocking rate.

FRTS doesn't take into account the flags and cyclic redundancy checks (CRCs) in the shaping algorithm. Because a flag and CRC need to be added for every fragment, on lower-speed links (768 kbps and below), you should shape to a value slightly lower than the actual CIR to account for the flags and CRCs. For example, on a 128-kbps Frame Relay PVC, the recommended frame-relay CIR value to use will be 124,872 bps with a Bc of 1250 bits per committed time window (Tc).

PVC Speed

Fragment Size (for 10ms delay)

Recommended CIR

Recommended Bc (CIR/100)

56 kbps

70 bytes

52968 bps

530 bits

64 kbps

80 bytes

60952 bps

610 bits

128 kbps

160 bytes

124872 bps

1250 bits

256 kbps

320 bytes

252832 bps

2530 bits

512 kbps

640 bytes

508816 bps

5090 bits

768 kbps

960 bytes

764940 bps

7560 bits

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