SUMMARY FastEthernet MTU

From: Eduardo E. Silva (
Date: Wed Dec 18 1996 - 18:06:34 CST

Many thanks to all who responded.

Original question
On Mon, 16 Dec 1996, Eduardo E. Silva wrote:

What is the MTU for FastEthernet (be0) on a SPARC1000E ? I know Ethernet is
at 1500, but since FastEthernet is ten times faster. Maybe it needs to be set
as FDDI (4352?).

** The answer NO: use 1500 for Ethernet and FastEthernet.

Summary of replies
From: "Trevor Paquette" <>

The packet size etc stays the same.. your transport layer is faster.

From: (poore)

Nope. Fast Ethernet uses the same MTU. The only difference in the spec. is
that the InterFrameGap is 1/10th the duration.

From: Chris Liljenstolpe <>


        MTU has nothing really to do with speed (a protocol will be the
best performer at the MTU it was designed for, loose performance with a
smaller MTU, and not work at a larger MTU). FastEthernet's design was to
keep as much of the Ethernet spec as possible, just bump the speed,
therefore, the MTU is still 1500.

From: "David Sinn" <>

The MTU for Ethernet (fast or normal) is exactly the same.

The timing is the only change.

From: "Matt R. Johnson" <>

MTU should still be 1500, data is just transmitted faster.

From: Benjamin Cline <>

I'm pretty sure the mtu for fast ethernet is the same as that for
ethernet. Fast ethernet doesn't send bigger chunks (like FDDI) it just
sends them faster.

From: (Avi J. Levin)

100BT packets still do not exceed 1500 bytes, they are just send faster.

From: Fernando Frota Redigolo <>

The MTU for FastEthernet is just the same: 1500; the only diferences
between ethernet and fastethernet are the transmission speed (10x faster)
and the network span (roughly 10x smaller).

From: Steve Phelps <>

nope. Fast Ethernet frames are identical in every respect to 'slow' ethernet

From: Tony Jago <>

 No, 1500 is correct.

From: Glenn Satchell - Uniq Professional Services <>

The packets are the same size to make it compatible with the older
10-base-T. It's just that they can be transmitted 10 times faster :-)

From: "Daniel J Blander - Sr. Systems Engineer for ACS" <Daniel.Blander@ACSacs.Com>

No, MTU is the Mean Transfer Unit (probably not a technically correct
translation of the TLA, but gets the point accross). It sets the
allowable size of the frame that can be transmitted for the given
network protocol. FDDI has a higher MTU (4000+ range), but FastEthernet
and Ethernet are the same, as will be GigabitEthernet. The frame size
does not change, only the frequency of the signal changes (Hz) that the
line is running at. FastEthernet runs at ten times the frequency as

ATM uses a 53 byte cell, so in realtive terms, its MTU is 53bytes (424)

Thanks!                                                            / <_> \
                                                                  /^^/ \^^\  
-Ed                                                                 /___\

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