brctl MTU problem over DSL

From: marian stagarescu (mstagarescu@telogy.com)
Date: Thu Jul 18 2002 - 21:04:02 EST


asymmetrici have a linux-bridge like below:

                          br0
                  |br2684 |
--DSL line---|atm eth|------LAN

I run a bridge on top of a regular eth interface and an
ATM interface which appears to the bridge as an ethernet interface (using
br2685 encapsulated ethernet over atm/aal5).

ifconfig will show MTU 1500 on all devices (br0, eth0, nas0 (created by
br2685ctl)

Now here is the problem: MTU

from LAN if I do a ping with packet size over 1428 bytes doesn't do it.
an ftp get from lan will also break if the file is not too small to fit under
1428. interesting an ftp-put from lan side will work with big files ( MTU ?)

how is this fixable ?

moreover if this box runs in routed mode

                                 IP
---internet---dsl---|br2684 eth0|---LAN

this works in terms of data transfers both ways with large packet sizes with
pppoe over br2684 (over atm) or directly pppoa over ATM.

i remember that this MTU was a problem with pppoe (1492 and not 1500 MTU).

my device being a bridge will not participate in any path mtu discovery or
send ICMP fragmentation required messages back an originating server
(like the ftp server above). I wonder how it works in the ftp-put mode ?

thanks,
marian

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