[Openswan Users] Re: MTU problems

Peter McGill petermcgill at goco.net
Mon Jun 12 09:38:23 CEST 2006

> Its seems the max MTU is 1492, and I have had to set this on all windows
> and Linux boxes to make the link useable,

1492 is the MTU for PPPoE links, which are often used by ISPs, particularly
with xDSL. Are you using PPPoE to connect to your ISP? If so this is why 
MTU is 1492. The other possibilities are that one of the other routers in 
internet route between your hosts has this MTU either because it is using 
or is perhaps simply misconfigured... In either case you will likely have to 
with the 1492 MTU, it's not so bad though, just set your MTU's accordingly.

If your not using PPPoE, you may be able to determine which router is 
the problem, but it's unlikely that the owner will fix it, assuming that it 
is a
misconfiguration. You can attempt to locate the router by resetting your MTU
to 1500 and doing some large traceroutes (packet size > 1492), the 
command will vary depending on your local traceroute program, but be sure to
set the don't fragment option and increase the packet size.

Peter McGill
Software Developer / Network Administrator
Gra Ham Energy Limited 

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