If a packet is sent over a link, the packet delayed due to serialization. Bigger packets need more time to get over the link, than smaller packets. If a 1500 bytes packet is in the transit, a small 150 bytes packet has to wait.
Here LFI can be used to solve the problem of bigger packets blocking smaller, for example voice packets, to long, and let them being sent before the whole 1500 bytes packet is completely being sent over the link.
The definition of packet is, that it includes the Layer 3 header information and the end-user-data, namely the payload. A frame is a packet, but includes also Layer 2 information header and trailer.
While fragmenting frames, the router will chop the 1500 byte frame possibly into two frames, which had to include again the header and trailer information in each new fragmented frame.
Make sure you configure „frame-relay fragment“ on both sides of the PVC. If not, one side will definetly have problems in recognizing the fragmented traffic. If only one side fragments the traffic and the other side will not defragment it, it will just drop all fragmented traffic coming over the DLCI.
A value of 80 kpbs recommended fragmentation size to every 64 kbps bandwidth. So 256 kbps interface bandwidth, means a fragment size of 320 kbps on this link.
Note: For interleaving to work, both fragmentation and the low-latency queueing policy must be configured with shaping disabled.
access-list 101 match ip any host 188.8.131.52
match access-group 101
interface serial 1/0
ip address 184.108.40.206 255.0.0.0
frame-relay fragment 80 end-to-end
clock rate 128000
service-policy output llq
Show and debug commands:
R5#sh frame-relay fragment interface s1/0 501
fragment size 200 fragment type end-to-end
in fragmented pkts 4511 out fragmented pkts 86
in fragmented bytes 109183 out fragmented bytes 10797
in un-fragmented pkts 162 out un-fragmented pkts 88
in un-fragmented bytes 10808 out un-fragmented bytes 5952
in assembled pkts 1053 out pre-fragmented pkts 130
in assembled bytes 94707 out pre-fragmented bytes 16317
in dropped reassembling pkts 0 out dropped fragmenting pkts
in DE fragmented pkts 4511 out DE fragmented pkts 0
in DE un-fragmented pkts 162 out DE un-fragmented pkts 0
in timeouts 0
in out-of-sequence fragments 0
in fragments with unexpected B bit set 0
in fragments with skipped sequence number 0
out interleaved packets 0
R5#sh frame-relay fragment
interface dlci frag-type size in-frag out-frag dropped-frag
Se1/0 501 end-to-end 200 4519 86 0
Se1/0 502 end-to-end 200 0 0 0
Se1/0 503 end-to-end 200 0 0 0
Se1/0 504 end-to-end 200 0 0 0
Se1/0 513 end-to-end 200 0 0 0
R5# debug frame-relay fragment interface s1/0 501
*Mar 1 04:33:08.866: Serial1/0(o): dlci 501, tx-seq-num 125, B bit set, frag_hdr 03 B1 80 7D
*Mar 1 04:33:08.870: Serial1/0(o): dlci 501, tx-seq-num 126, no bit set, frag_hdr 03 B1 00 7E
*Mar 1 04:33:08.874: Serial1/0(o): dlci 501, tx-seq-num 127, E bit set, frag_hdr 03 B1 40 7F
Cisco QOS, Second Edition, Exam Certification Guide