L2TP + IPSec, best way to avoid fragmentation

[m.sobolev]

Hi there!
Once again, let`s discuss VPN and MTU What we have? Ethernet LAN, L2TP to ISP and IPSec with AES crypto in tunnel mode. So protocols overhead is 40+73 bytes (L2TP + ESP). What is the best way to avoid fragmentation for all IP protocols on ...

[Edison Ortiz]

The best way to avoid fragmentation is for the MTU size on the communication to be less than the available MTU on the transit interfaces.

With TCP, you can use MSS and PMTUD as described on this article

http://www.cisco.com/en/US/tech/tk827/tk369/technologies_white_paper09186a00800d6979.shtml

As for UDP and other IP protocols that do not regulate their size based on MTU discovery, their packet will manually need to be reduced from the source and face fragmentation or drops while traversing the transit network.

[m.sobolev]

What is the best way to set MTU? Choices are: Fe4 (WAN), Virtual-PPP1 (L2TP session interface) or Vlan1 (LAN).