Link Fragmentation and Interleaving (LFI) is an enhancement to Multilink PPP (MLP) that fragments packets at the Layer 2 (PPP) level. Fragmenting at the IP layer, as with MTU reduction, forces the addition of a new 20 byte IP header and an 8 byte PPP header. However, fragmenting at the data link (PPP) layer only forces generation of an 8 byte PPP header, which greatly increases the efficiency of the link.

You must use the LFI functionality instead of MTU manipulation when transmitting IP telephony packets over PPP links. As with MTU, you must size the packets so that the serialization delay is approximately 10 ms or less.