Overview

Because the hardware transmit queue (TxQ) is a FIFO queue, having a large packet such as an FTP packet in front of a small packet like a VoIP packet can cause excessive delay for the VoIP packet. Link fragmentation and interleaving (LFI) allows the large packets to be fragmented into smaller fragments, and then the small unfragmented packets are interleaved in between the fragments. LFI reduces the delay and jitter of small packets such as VoIP packets over a WAN link.

This lesson describes the configuration and monitoring of two different LFI mechanisms supported in Cisco IOS software, Multilink PPP LFI (MLP LFI) and Frame Relay Fragmentation (FRF. 12).

Objectives

Upon completing this lesson, you will be able to configure LFI to improve bandwidth efficiency and reduce delay. This ability includes being able to meet these objectives:

■ Identify the different options available for link fragmentation

■ Given a list of link speeds and a specific delay requirement, determine the proper fragment size to use at each link speed and the typical delay requirements for VoIP packets

■ Identify the Cisco IOS commands required to configure MLP LFI

■ Identify the Cisco IOS commands used to monitor MLP LFI

■ Explain when FRF. 12 can be used and how FRF. 12 affects VoIP packets

■ Identify the Cisco IOS commands required to configure FRF. 12

■ Identify the Cisco IOS commands required to monitor FRF. 12

Fragmentation Options

This topic describes the different options that are available for link fragmentation.

This topic describes the different options that are available for link fragmentation.

LFI is a Layer 2 technique, in which all Layer 2 frames are broken into small, equal-sized fragments, and transmitted over a link in an interleaved fashion. LFI reduces delay and jitter by expediting transfer of smaller frames through the hardware TxQ.

There are three LFI mechanisms implemented in Cisco IOS software:

■ MLP LFI: By far the most common and widely used form of LFI

■ FRF.12 Frame Relay LFI: Used with Frame Relay data connections

■ FRF.11 Annex C LFI: Used with Voice over Frame Relay (VoFR)

Note For configuration information on FRF.11 .C, refer to the latest Cisco IOS documentation.

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