SUMMARY: FDDI / Ethernet MTU problems (tuning ??)

From: Christian Lotz (
Date: Sat Jan 17 1998 - 09:25:41 CST

Dear Sun Managers,

first of all ... thanks to:

Michel Pilon <>
Rick Reineman <>
Ira Childress <>
Mark A. Baldwin <>
Frank Cusack <>

My posting:
> I have a bunch of Solaris Server in our FDDI Backbone. All of the
> clients are connected to ethernet segments. As far as i know is the
> MTU for ethernet 1500 and for FDDI 4352 (default). This means that
> all packets send from the FDDI segment that are bigger than 1500 Bytes

> get fragmented by the router. Would it no be more reasonable to limit
> the MTU for FDDI to 1500 like ethernet so that there is no
> needed on the router connecting FDDI to ethernet. Do I need
> mtu_path_discovery if i limit the MTU to 1500 ??
> There are also some problems appearing on connections via leased lines

> since the servers are in fddi backbone (fragmentation ???) ...
> Has anybody experience with this subject.

Almost everybody pointed out to reset all the Interfaces to MTU 1500.
There are known problems of connections between FDDI (MTU 4352) and
Ethernet (1500). Normally the mtu path discovery should solve that
problem ...

This is what i got reading the manuals ...a packet ist sent with the
configured MTU with the don't fragment bit set, and the router should
sent some icmp error to the source host if the packet needs to be
frgamented, and the next smaller MTU size is used during the next trial
until the packet gets to the destination without fragmentation. This MTU
size is hold in the cache for about 20 minute, and the procedure happens
again - I hope I got it right ...).

But it doesn't worked for me so i adjusted all the MTUs to 1500 and
everything worked out well for me .... If I have the time I try to track
down the problem and I will post the solution ...

Once again ... thanks to everybody for the help ...


Christian Lotz         Unix System Technologist
                  Suelmerstrasse 30 - D 74072 Heilbronn
Fon: 07131 628536           Fax: 07131 628537        Cel:0172 7408605

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