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Understanding MLPPP Link Fragmentation and Interleaving

Priority scheduling on a multilink (MLPPP) bundle determines the order in which an output interface transmits traffic from an output queue. The queues are serviced in a weighted round-robin fashion. But when a queue containing large packets starts using the MLPPP bundle, small and delay-sensitive packets must wait their turn for transmission. Because of this delay, some slow links can become useless for delay-sensitive traffic.

Link fragmentation and interleaving (LFI) solves this problem by reducing delay and jitter on links by fragmenting large packets and interleaving delay-sensitive packets with the resulting smaller packets for simultaneous transmission across multiple links of a MLPPP bundle.

Figure 1 shows how LFI processes packets.

Device R0 and Device R1 have LFI enabled. When Device R0 receives large and small packets, such as data and voice packets, it divides them into two categories:

  • All voice packets and any other packets configured to be treated as voice packets are categorized as LFI packets and transmitted without fragmentation or an MLPPP header.

  • The remaining non-LFI (data) packets are fragmented or unfragmented based on the configured fragmentation threshold. Packets larger than the fragmentation threshold are fragmented. An MLPPP header (containing a multilink sequence number) is added to all non-LFI packets, fragmented and unfragmented.

Fragmentation is performed according to the fragmentation threshold that you configure. For example, if you configure a fragmentation threshold of 128 bytes, all packets greater than 128 bytes are fragmented. When Device R1 receives the packets, it sends the unfragmented voice packets immediately but buffers the packet fragments until it receives the last fragment for a packet. In this example, when Device R1 receives fragment 5, it reassembles the fragments and transmits the whole packet.

The unfragmented data packets are treated as a single fragment. Device R1 transmits the unfragmented data packets as it receives them and does not buffer them.