Re: brctl MTU problem over DSL

From: Stephen Hemminger (shemminger@osdl.org)
Date: Fri Jul 19 2002 - 11:11:19 EST


Bridging between networks with different MTU's is always going to be a
problem. You can of course set a smaller mtu with ifconfig.
Ideally, the bridge would be smart enough to send back ICMP packets so
MTU discovery would work, but the bridge probably doesn't understand IP.

On Thu, 2002-07-18 at 19:04, marian stagarescu wrote:
> 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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