PPP MTU on an Ethernet link.

[peasthope@shaw.ca]

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
    
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?  Is there any 
practical way to get 1500?

Thanks,          ... Peter E.





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