/mcpQuality of Service for Voice over IP Fragmentation and Interleaving 12 QoSVoIP.mif Table 2 shows recommended fragment sizes for various link speeds based on the 10-ms rule. Table 2 Link Speed and Fragmentation Size Note The packet fragmentation size should never be lower than the VoIP packet size. Also, you should never fragment VoIP packets—fragmenting VoIP packets can cause numerous call setup and quality problems. Three link fragmentation and interleaving (LFI) mechanisms are available. Table 3 lists their benefits and limitations. Link Speed (kbps) Fragmentation Size (Bytes) 56 70 64 80 128 160 256 320 512 640 768 960 1024 1280 1536 1920 (No fragmentation is required if the fragment size is larger than the link MTU size. For example, for a T1 link with a 1500-byte MTU, the fragment size is 1920 bytes; therefore, no fragmentation is required.) Table 3 Link Speed and Fragmentation Size LFI Mechanism Description Benefits Limitations MTU Fragmentation with WFQ Interface-level command to change MTU size or IP MTU size. Used to fragment large IP packets to specified MTU size. LFI uses WFQ to interleave real-time packets in between the fragments. Simple configuration. Fragments are reassembled only by receiving application; therefore use of network is inefficient. Only IP packets with Don't Fragment (DF) bit not set can handle fragmentation well. Highly-processor intensive. Not recommended. Multilink Point-to-Point Protocol (MLP) Link Fragmentation and Interleaving (LFI) On point-to-point serial links, MLP must first be configured, then a fragmentation size must be set in milliseconds. Interleaving must also be enabled on the multilink interface. Packets are fragmented on one end of link and reassembled at the other. Several links can be combined to act as a large virtual pipe. Only available on links configured for PPP. Solutions for PPP over Frame Relay or PPP over ATM also are supported in Cisco IOS Release 12.1(5)T or later releases.