R3102-R3103-HP 6600/HSR6600 Routers Layer 2 - WAN Configuration Guide

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CAUTION:
Disabling LFI also removes the user-configured settings of maximum LFI fragment delay and size.
On a low-speed serial link, packets of real-time interactive communications (such as Telnet and VoIP)
might be blocked or delayed if packets of other applications are also transmitted across the link. For
example, if a voice packet arrives when large packets are being scheduled and waiting to be transmitted,
it must wait until all the large packets have been transmitted. For the real-time applications, such as VoIP,
delays longer than 100 or 150 ms cause voice quality to drop dramatically and cannot be tolerated.
On a 56 kbps link, it costs approximately 215 ms to transmit a 1500-byte packet (the size of the MTU of
common links). To confine the delay of transmitting time-sensitive packets on low-speed links (such as 56
kbps frame relay channels or 64 kbps ISDN B channels) to an acceptable level, a method is required to
fragment larger packets and adding both the smaller packets and fragments of the large packet to an
output queue.
LFI reduces delays and jitters on low-speed links by fragmenting large packets into small fragments and
transmitting them along with small packets. The fragmented datagrams are reassembled at the
destination.
Figure 11 illu
strates the process of LFI. When large packets and small voice packets arrive at an interface
that is enabled with WFQ at the same time, the large packets are fragmented into small fragments, which
are then added to the queues along with the voice packets.
Figure 11 LFI
To configure LFI:
Ste
p
Command Remarks
1. Enter system view.
system-view N/A
2. Enter VT interface view or MP-group
interface view.
• interface
virtual-template number
• interface mp-group
mp-number
N/A
3. Enable LFI.
ip tcp vjcompress By default, LFI is disabled.
Voice packet
Large packet
Output queue
WFQ
Traffic
classifying
Fragmentation
WFQ