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From: robelr@ucs.indiana.edu (Allen Robel)
Newsgroups: comp.dcom.sys.cisco
Subject: cisco's FDDI MTU vs. RFC1188
Keywords: FDDI, MTU
Message-ID: <1991May21.215118.19401@bronze.ucs.indiana.edu>
Date: 21 May 91 21:51:18 GMT
Article-I.D.: bronze.1991May21.215118.19401
Sender: news@bronze.ucs.indiana.edu (USENET News System)
Organization: Indiana University
Lines: 22

A show interface on a cisco FDDI interface shows an MTU of 4470. 
RFC1188 specifies an MTU of 4352 octets for FDDI:

      Therefore, the MTU of FDDI networks shall be 4352 octets.  This
      provides for 4096 octets of data and 256 octets of headers at the
      network layer and above.  Implementations must not send packets
      larger than the MTU.

I don't think that there is any practical harm in this discrepancy
in our environment as we're just using the FDDI to connect
ethernets and I don't think that the cisco would ever generate
frames this large on its own.  But I was wondering if some
enlightened soul might shed some light on why the descrepancy
exists.  Too, I'm bored and couldn't think of a better way
to spend my time than to post this :-)

thanks,

Allen Robel                         robelr@mythos.ucs.indiana.edu 
University Computing Services       ROBELR@IUJADE.BITNET 
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