PPPOE, MTU, and boom.

Alex Rubenstein alex at nac.net
Wed Jul 18 07:24:22 UTC 2001

On Wed, 18 Jul 2001, Chris Wedgwood wrote:

> On Wed, Jul 18, 2001 at 07:57:23AM +0100, Stephen J. Wilcox wrote:
>     Could it be Path MTU/fragmentation issues on account of the low
>     MTU you have in combination with some ICMP issues where network
>     admins have some screwy setup?
> That's exactly what it is... a large (1500 byte probably) TCP datagram
> with the DF bit set will be heading towards his edge device which are
> PPPoE connected.

Great domain, btw (f00f.org, brings back memories of division)...

Anyway, a great stride was just made in my lab^H^H^Hbasement. First off,
it appears a 'ip mtu 1492' on the Virtual-Template of the IOS Router
aggregating the PPPOE session is not strictly enforced, especially with
windows. Another words,  I saw in the PPP debug that the MRU wasn't being

So, I found this really nifty utility, aptly named 'Mr. TCP':


It allows you to tinker with the Windows settings of TCP. I adjusted MTU
to 1400, wham-o, all the sites I listed started working; including, what I
didn't list before, the downloading of transactions via Quicken. Seems to
be a fixall.

-- Alex Rubenstein, AR97, K2AHR, alex at nac.net, latency, Al Reuben --
--    Net Access Corporation, 800-NET-ME-36, http://www.nac.net   --

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