[rescue] Cisco Catalyst 5000 FDDI Software

Zach Lowry zach at zachlowry.net
Tue May 27 21:31:21 CDT 2008

On May 27, 2008, at 6:33 PM, Dan Sikorski wrote:
> You have a newer supervisor and newer CatOS than I do, so I'm  
> suprised that this doesn't just work for you.

But likely an older version of the FDDI software on the FDDI module,  
which is bundled separately from the Supervisor software. Could you  
send a 'show module' for your FDDI module?

> 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.

I believe the Cisco FDDI module is supposed to handle the  
disassembly / reassembly for me. From [0]:

IP fragmentation allows the switch to fragment large FDDI IP frames  
(frames greater than 1514 bytes) into multiple smaller packets so  
that they can be transmitted on an Ethernet segment. IP fragmentation  
is enabled by default.

I don't recall ever being on a network where PMTU worked correctly,  
which could be because of something silly I've done repeatedly in  
breaking it. :)

[0] http://www.cisco.com/en/US/docs/switches/lan/catalyst5000/catos/ 



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