[c-nsp] l2tp xconnect problem with packets > 1430

Peter Rathlev peter at rathlev.dk
Thu Dec 8 09:48:56 EST 2011

On Wed, 2011-12-07 at 19:09 -1000, Antonio Querubin wrote:
> On Wed, 7 Dec 2011, Peter Rathlev wrote:
> > If the L2TPv2 packets cannot exceed 1500 bytes then the ICMP payload
> > size of an encapsulated packet cannot exceed 1430 bytes, assuming
> > you're not transporting a 802.1Q frame.
> Understood.  What I don't understand is why the large pings (in this
> case large is anything > 1430) will sometimes work and sometimes not.
> There is no packet loss between the two routers and pmtud works
> between them.  They can ping each other with large packets all day.
> Not so the xconnected hosts.

What Gert said. What is the distribution of forwarded/non-forwarded
packets? Maybe there's a timeout on the PMTUD resettings things, and
maybe the router accepts fragmenting for a short while here.

It shouldn't work with an MTU larger than 1430 though. It's an error
that it does. ;-)


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