Thanks! I had just never stopped to think too deeply about *why* different
network-layer protocols would pick one ethernet frame type over another.
John
> At 07:38 AM 2/5/01, John Neiberger wrote:
> >While studying for CIT, I noticed something that had never occurred to
me
> >before. The default ethernet frame type on a Cisco router is
Ethernet_II,
>
> The default frame type depends on the payload.
>
> The default for IP is Ethernet V2 because the IP industry never adopted
> anything newer at the data-link layer. (They did adopt new physical-layer
> IEEE 802.3 standards.) Ethernet V2 has dest, source, and EtherType. If
you
> were to change the frame type on the routers, you would have to change it
> on all IP hosts too, which would be a pain. Most operating systems
(Windows
> 9x, Window NT, SunOS, Mac OS, etc.) default to Ethernet V2 for IP also.
>
> If you use AppleTalk Phase 2, the default frame type is 802.3 with 802.2
> and SNAP. That's because all Macintoshes and other AppleTalk devices
> default to that frame type for AppleTalk also. (Phase 1 was Ethernet V2,
by
> the way.)
>
> If you use Novell, the default is Novell "raw," aka Ethernet_802.3 which
> has dest, source, length, immediately followed by the IPX header which
> starts with an XNS checksum, which isn't used so it's always FFFF.
>
> The Novell default may have changed. I know Novell has been wanting to
get
> with the rest of the world, plus they have been talking about actually
> using the checksum, which means they can't use the raw format. Also the
raw
> format is kind of ugly because a "raw" frame arrives at a station
> configured for 802.3 with 802.2, the FFFF looks like a global LLC (802.2)
> SAP, which means "give this frame to all services!.
>
> Priscilla
>
> >but the only physical interface specified by Ethernet version 2 is
50-ohm
> >coax, IIRC, similar to 10base5 On 10baseT or 100baseTX interfaces,
which
> >are on every router I've ever worked with, why is the default frame type
not
> >IEEE 802.3?
> >
> >Ethernet_II only has a type field, while IEEE 802.3 frames include 802.2
> >information. What sorts of functionality would be available through the
use
> >of that frame type that are not available with Ethernet_II?
> >
> >In IP-only environments, would there be a good reason to change to a
> >different frame type, or would we only benefit from a different frame
type
> >in a non-IP environment or mixed environment?
> >
> >Thanks,
> >John
> >
> >
> >
> >
> >
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>
> ________________________
>
> Priscilla Oppenheimer
> http://www.priscilla.com
>
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