On Mon, 2010-03-29 at 13:32 -0700, peasthope@shaw.ca wrote: > Observed Phenomenon > The interface from a Squeeze Linux router to > an ISP cable modem, configured with this line > in /etc/network/interfaces, has MTU = 1500. > iface eth0 inet dhcp > Good; just as expected. > > For Lenny, the same /etc/network/interfaces > yields MTU = 576. Very odd. This value > should exist for a ppp link and not for an > eth link. > > If a D-Link DI-614+ is inserted between Lenny > and the cable modem, then MTU = 1500. > > Background Factors > I prefer to keep Lenny on the machine until > Squeeze becomes more reliable. Configuration > will be more difficult if the D-Link router > remains permanently. > > Failed Solution > According to > http://www.debian.org/doc/manuals/reference/ch05.en.html#_setting_mtu > I adjusted /etc/network/interfaces to this. > iface eth0 inet dhcp > hostname "joule" > pre-up /sbin/ifconfig $IFACE mtu 1500 IIRC, you should be able to tweak the MTU after the link comes up. Perhaps trying 'post-up' instead. > > Still the wrong MTU is set. > peter@joule:~$ ip link show eth0 > 3: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 576 qdisc pfifo_fast state UNKNOW > N qlen 1000 > link/ether 00:50:ba:52:79:1c brd ff:ff:ff:ff:ff:ff > > Questions > Can anyone explain why the dhcp negotiation > in Lenny results in MTU = 576? It may be a path MTU discovery. -- Matt Zagrabelny - mzagrabe@d.umn.edu - (218) 726 8844 University of Minnesota Duluth Information Technology Systems & Services PGP key 4096R/42A00942 2009-12-16 Fingerprint: 5814 2CCE 2383 2991 83FF C899 07E2 BFA8 42A0 0942 He is not a fool who gives up what he cannot keep to gain what he cannot lose. -Jim Elliot
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