[rescue] Cisco Catalyst 5000 FDDI Software

Dan Sikorski me at dansikorski.com
Tue May 27 18:33:22 CDT 2008

Zach Lowry wrote:
> I have a Supervisor 3, and CatOS 6.4(22). It *recognizes* the FDDI 
> module, but the bridging doesn't seem to be translating the fram size 
> between the 4352 MTU FDDI segment and the 1500 MTU 10/100 segment. 
> When I mount something via NFS with the client on  FDDI and the server 
> on 10/100, the server never sees any packets, nor does the SPAN port 
> on teh 10/100 segment. No interface in the loop shows any errors, 
> switch side or client side, except on the client.
> caterpillar.in.zachlowry.net -> cheshire.in.zachlowry.net UDP IP 
> fragment ID=64443 Offset=0    MF=1
> caterpillar.in.zachlowry.net -> cheshire.in.zachlowry.net UDP IP 
> fragment ID=64443 Offset=4328 MF=0
> If I set the MTU on the client to 1500, everything works fine. So, it 
> appears to me that the bridge on the FDDI module is dropping the 
> frames. That, or the C1400 concentrator is. But I can't find any 
> documentation on what size frames the C1400 will support.
You have a newer supervisor and newer CatOS than I do, so I'm suprised 
that this doesn't just work for you.  I think there are others that 
could offer a more complete answer here (if i'm misunderstanding, or 
just plain wrong on anything, please correct me), but my understanding 
is that the catalyst 5000 does not actually fragment the >4k packets 
from FDDI to 1500byte packets for the ethernet segment.  Doing so would 
cause all sorts of mess to the higher layer protocols contained in the 
frame.  Instead, what should happen is that a host on the FDDI network 
should do path MTU discovery to figure out what size packets to sent 
out, and when talking to a host on the ethernet network, only send out 
1500byte packets.  I don't have any hosts on my FDDI network right now, 
so i can't do any meaningful testing, but as i recall, you should be 
able to do some testing with ping -s and determine if path mtu discovery 
is working properly.

    -Dan Sikorski

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