SUMMARY: Problem with FDDI/SS2000/NFS

From: Dave Haut (
Date: Fri Jan 12 1996 - 12:40:15 CST

Hi again !!

2 days ago, I asked:


I have a SPARCCenter 2000 running Solaris 2.4 with all of Mandatory
patches installed. The SC2000 has an interphase FDDI connection to my
network. The SC2000 is acting as an NFS server. All of my NFS clients
have 10 base T connections to my network.

The problem that I am having is that when I NFS mount partitions
from this SC2000 to Sun's running SunOS 4.1.x, the performance is VERY
slow and I keep getting "NFS server xxx not responding" and
NFS server xxx ok" messages on the console on these Sun 4.x machines.

If I NFS mount partitions from this SC2000 to Sun's running Solaris 2.x,
I have no problems at all.

If I remove the the SC2000 from the FDDI and connect it VIA a 10 base T
connection to my network, The problem goes away.

I don't think that this is a network problem since my Solaris 2.x machines
are not experiencing any problems. What I suspect is ether the interphase
FDDI card in my SS2000, or perhaps there is some NFS kernel parameter I can
adjust in my /etc/system file.

Does anyone out there have any suggestions ?? Has anyone else experienced
a similar problem ???

Thanks for the Help !!

Well basically, I had two problems.

First , because of the difference in MTU sizes for FDDI (~4500 bytes ) vs
ethernet (~1500 bytes ) my switches must be able to fragment the larger
FDDI packets down to ethernet packets. Apparently there is a "Don't
Fragment" bit that is part of the FDDI datagram packet and some
switches/routers look at and act upon. Apparently, by default the bit is set
to "on" in Solaris and either off or n/a in SunOS 4.x. This was part of my

I have Cabletron 9E133-36 microlan switches that apparently look at this "Don't
Fragment" bit. Thanks to and bobr@cassie.Sugar-land.Wireline.SLB.COM who suggested entering the following command on the Sun which
disables this bit:

/usr/sbin/ndd -set /dev/ip ip_path_mtu_discovery 0

That seemed to solve the problem woth Sun's that I had connected to the
Cabletron 9E133 cards.

But I still had the problem on the Cabletron 9E312 Ethernet switch cards.
I called Cabletron and apparently these cards are incapable of fragmenting
FDDI datagrams down to Ethernet datagrams, so I bit the bullet and just
changed the MTU size on my FDDI to 1500 bytes which universaly solved the

Thanks to the following for your inquiries:


  _ /| Dave Haut
  \,o.O' Sys Admin
  =(___)= EXAR Corporation
             (510) 438-7570

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