I made an half asleep typo. Anything small was not quite what I meant.
Teunis.
On Sunday, December 03, 2000 at 05:31:49 PM, Priscilla Oppenheimer wrote:
> The Ethernet standards (Blue Book and IEEE 802.3) say that the maximum
> Ethernet frame size is 1518 bytes, counting the dest and src addresses,
> type/length field, and FCS. But, theoretically, there's no reason it
> couldn't be bigger, and some interfaces accept bigger frames. They have to
> for ISL, as you mentioned.
>
> I asked Bob Metcalfe where 1518 bytes comes from. Seriously, I met him at a
> party many years ago and hit him with this question. His answer, "Hmm, well
> I really don't know!"
>
> Some sort of maximum is necessary, of course, in order to assure fairness.
> The myths about the maximum having something to do with making CSMA/CD work
> correctly are inaccurate, (not that anyone said that in this thread). The
> myths are a mistake by analogy. The minimum size frame is necessary for
> CSMA/CD to work correctly.
>
> Priscilla
>
> At 09:22 AM 12/4/00, Tony van Ree wrote:
> >Hi all,
> >
> >I was under the impression that an ethernet frame had a maximum size of
> >1500 bytes. This did not include the addresses or the FCS. To increase
> >the MTU beyond that size would indeed create a giant on ethernet. As it
> >is you will find some devices give an error message when including an ISL
> >header. (A gig port on a 2984 is a good example) In the case of the 2984
> >the data still goes through ok but when you get a packet over 1484 bytes
> >an whack it through the port the error rate rises. Anything small is ok.
> >
> >It does seem to me that the basic ethernet standards define the max MTU
> >pretty well.
> >
> >I could however have mis understood all these years.
> >
> >Thanks,
> >
> >Teunis,
> >Hobart, Tasmania
> >Australia.
> >
> >On Saturday, December 02, 2000 at 12:25:13 AM, Kevin Wigle wrote:
> >
> > > there is a "mtu" command.
> > >
> > > In a somewhat similar situation - I had built a circuit using lan emulation
> > > that terminated on a bvi on a 7505.
> > >
> > > The bvi had an ip address and placed into an OSPF area.
> > >
> > > I'm not aware of the defaults with a router that has both ATM and ethernet
> > > but I didn't take notice of the mtu that the bvi had - I assumed that
> > it was
> > > just like an ethernet interface........
> > >
> > > On the distant end of the lan emulation was an ordinary router with an
> > > ethernet interface and it was also configured into the OSPF area.
> > >
> > > However, OSPF didn't work. debugging showed that an adjacency wasn't being
> > > formed.
> > >
> > > Turning up yet more debugging - finally an error about "mtu size not equal"
> > > was noticed.
> > >
> > > sure enough, the bvi had the ATM mtu of 4470..........
> > >
> > > using the mtu command on the bvi interface "mtu 1500", the adjacency formed
> > > immediately and all was well.
> > >
> > > so, my guess would be that a fast ethernet probably has the same "mtu"
> > > command, try it out.
> > >
> > > Kevin Wigle
> > >
> > > ----- Original Message -----
> > > From: "Darren Ward" <[EMAIL PROTECTED]>
> > > To: <[EMAIL PROTECTED]>
> > > Sent: Friday, 01 December, 2000 22:02
> > > Subject: Fast Ethernet MTU Size
> > >
> > >
> > > > Hi All,
> > > >
> > > > Is it at all possible to raise the MTU size on Fast Ethernet?
> > > >
> > > > I'll give you all a scenario....
> > > >
> > > > ATM on one side with an MTU of 4470, fast ethernet connecting the two
> > > > routers, Gigabit Ethernet the other side with an MTU of 4470.
> > > >
> > > > How can I raise the MTU of a Full Duplex Fast Ethernet Connection above
> > > > 1500?
> > > >
> > > > I assume it's not possible when going through switched as they will show
> > > > every packet as a giant but in a router to router cross-over connection
> > > > I was hoping there was some way to keep the MTU static across the path
> > > > rather than force the routers to fragment and re-assemble (of course the
> > > > destination re-assembles).
> > > >
> > > > Darren Ward
> > > > CCNP, CCDP, CCIE Wannabee
>
>
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
>
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