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
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