Packet Fragmentation; L2Tp Platform Considerations - Juniper JUNOSE SOFTWARE 11.2.X - BROADBAND ACCESS CONFIGURATION GUIDE 7-20-2010 Configuration Manual

Packet Fragmentation

L2TP Platform Considerations

Copyright © 2010, Juniper Networks, Inc.
The E Series router supports the reassembly of IP-fragmented L2TP packets. (For more
information, see the IP Reassembly for Tunnels chapter in JunosE IP Services Configuration
Guide.) However, it is preferable to prevent fragmentation within L2TP tunnels because
of the effects of fragmentation and reassembly on performance.
To prevent fragmentation, PPP LCP negotiation of the maximum receive unit (MRU)
may be used to determine a proper maximum transmission unit (MTU). However, the
normal automatic method of determining the proper MRU to negotiate (by evaluating
the MRU of all lower layers in the interface stack) is not adequate for L2TP. The initial
LCP negotiation between PPP in the client and the LAC is inadequate because it does
not cover the entire extent of the eventual PPP session that travels all the way from the
client to the LNS. Furthermore, even if PPP in the LNS chooses to renegotiate the MRU,
it has no way to determine the proper MRU, since it does not know the minimum MRU
on all of the intervening links between it and the LAC.
To overcome the inadequacy of normal determination of the MRU under such
circumstances, you can configure the PPP MRU size by using the ppp mru command in
Profile Configuration mode, Interface Configuration mode, or Subinterface Configuration
mode. Use Profile Configuration mode for dynamic PPP interfaces, and Interface
Configuration mode or Subinterface Configuration mode for static PPP interfaces.
When you specify the size, you need to take into account the MRU for all possible links
between the LAC and the LNS. You must also take into account the L2TP encapsulation
that is added to all packets entering the tunnel.
For example, if the link between the LAC and LNS with the lowest MRU were an Ethernet
link, the following calculation applies:
Minimum link MRU
L2TP encapsulating IP header
L2TP encapsulating UDP header
Maximum L2TP header (assumes a maximum of
16 bytes of Offset Pad)
MRU size to specify
If the smallest intervening link is an Ethernet link, specifying ppp mru 1442 at either the
LAC or LNS guarantees that no fragmentation will occur within the L2TP tunnel.
For information about modules that support LNS and LAC on the ERX7xx models, ERX14xx
models, and the ERX310 Broadband Services Router:
See ERX Module Guide, Table 1, ERX Module Combinations for detailed module
specifications.
Chapter 11: L2TP Overview
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