Juniper JUNOSE 11.1.X - LINK LAYER CONFIGURATION 4-7-2010, Руководство по настройке

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End-to-End Fragmentation and Reassembly
The fragmentation and reassembly feature reduces excessive delays of Frame Relay
packets by breaking them up into smaller fragments and interleaving them with
real-time frames. By doing this, real-time and non-real-time data frames can be
carried together on lower-speed links without causing excessive delays to the real-time
traffic. On receiving the smaller fragments by the peer interface, the fragments are
reassembled into their original packet. For example, short delay-sensitive packets,
such as packetized voice, can race ahead of larger delay-insensitive packets, such as
common data packets.
E Series routers support end-to-end fragmentation according to the FRF.12
Implementation Agreement standard. Unlike UNI and NNI fragmentation, end-to-end
supports fragmentation only at the endpoints. End-to-end fragmentation and
reassembly are supported only on non-multilink Frame Relay interfaces on
cOC12/STM4 and CT3 12 FO modules.
You configure end-to-end fragmentation at the Frame Relay subinterface level.
Fragmentation is applied to all PVCs associated with the subinterface. In most cases,
fragmentation and reassembly are used together. Fragmentation and reassembly,
however, can be configured separately for each map class.
For additional information, see Frame Relay Forum—Frame Relay Fragmentation
Implementation Agreement, FRF.12 (December 1997).
Frame Fragmentation
When you enable fragmentation, you can specify a maximum payload size of the
resulting fragments. If the maximum payload size is not specified, the default value
of 52 bytes is used. When enabled, fragmentation begins when the portion of the
packet that has not been transmitted in previous fragments exceeds the configured
maximum payload size. The fragmentation process continues until the entire packet
has been transmitted. Frames that do not exceed the configured maximum payload
size are not fragmented.
If you disable fragmentation, all packets transmitted by the Frame Relay subinterface
are transmitted intact.
Frame Reassembly
When reassembly is disabled and a data frame is received, a few scenarios may
occur:
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If the frame is not a fragment, it is forwarded normally.
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If the frame is a fragment and the upper interface is IP (that is, the interface
above the Frame Relay subinterface), then the fragment is immediately discarded.
If you enable reassembly, then received fragments undergo the reassembly process.
Packets that are not fragments are forwarded as normal.
End-to-End Fragmentation and Reassembly
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Chapter 2: Configuring Frame Relay