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From: William "Chops" Westfield <BILLW@mathom.cisco.com>
Newsgroups: comp.dcom.sys.cisco
Subject: Re: cisco's FDDI MTU vs. RFC1188
Message-ID: <35274@boulder.Colorado.EDU>
Date: 22 May 91 00:34:34 GMT
Sender: daemon@boulder.Colorado.EDU
Lines: 19


    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.

The RFC reserves some space for "expanding the MAC header" (what a bad
idea (expanding it, not reseving the space)).  The cisco implementation
pre-dates RFC1188, and is based on RFC1103, which specifies 4470.

Of course, you can change the MTU on the cisco to whatever you would like,
assuming it is smaller than that actually supported by the media...

Bill Westfield
cisco Systems.
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