Ethernet can indeed run at a higher MTU as our discussion showed, in fact we are
trying to get hold of some 2FE cards to do just that, we already run MTU4470 on our
gig to gig links but remember these do not go through any switches but are router to
router only so fixed MTU ports are avoided.
Now a giant is only detected if the packet was over 4470 not 1500 (Have tested that
already).
However back to your point, you can't raise a normal ethernet or single port fast
ethernet card to more than 1500 bytes and most switches don't like a higher MTU either.
Still it is possible, does work and changes the sizes of giants automatically as the
MTU is modified.
Darren
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
> > >
> > > _________________________________
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