Subject: Re: MTUs
To: None <firstname.lastname@example.org, email@example.com>
From: der Mouse <mouse@Rodents.Montreal.QC.CA>
Date: 01/20/2000 15:56:02
> [MTU changes on Ethernet interfaces] is necessary (AFAIK) for
> machines running behind a firewall/gatway using PPPoE/PPP combination
> to connect through ADSL.
It shouldn't be necessary. If PMTU discovery is off, the too-large
packets should get fragmented normally; if it's on, a needs-frag ICMP
should be returned. Either way, no problem.
Except that's clearly not what you're seeing....
> So why is it that when I set the MTU for PPP in the options file to
> 1450, it still won't pass the traffic if the ethernet frame from the
> initiating machine is 1500 bytes long?
This sounds like a bug...unless the packets have their don't-fragment
bits set. Do they? If so, are need-frag ICMPs being sent back in
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