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Ethernet Encapsulation and 802.3

Posted on 2004-09-03
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Can someone kindly explain why 802.3 encapsulation is used for certain things, while ethernet II is showing up on some other things?

Here is what I mean. I ran ethereal and watched some traffic.

All of the CDP and STP traffic was utilizing 802.3 framing. I also noticed something called "LLC" everytime I saw the 802.3 framing method being used. What is the correlation?
http://mvpbaseball.cc/8023.jpg


Most of the other traffic was framed with Ethernet II.  I noticed the "LLC" was absent completely everytime the Ethernet II framing method was used.  
http://mvpbaseball.cc/e2.jpg

Thanks
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Question by:dissolved
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Don Johnston earned 500 total points
ID: 11974236
There are 4 different types of "Ethernet"

Version II (Cisco calls it ARPA)
IEEE 802.3 w/802.2 LLC (Cisco calls it SAP)
IEEE 802.3 snap (Cisco calls it SNAP)
Novell RAW (Cisco calls it novell-ether)

In the Cisco world:
CDP traffic uses SNAP frames.
STP BPDU's use 802.3 frames.
Most everything else is version II (arpa)

Novell RAW is IPX only so the only time you see that is in older Novell networks.

-Don
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Author Comment

by:dissolved
ID: 11974711
Why was CDP designed to use SNAP?  And why was STP designed to use 802.3?  Is there any real reason or is it just because the designers of these protocols felt like doing it that way.



Also, I have been noticing I have problems with my STP protocol.  When a CRC is being run on the FCS, it is coming up incorrect: (did I say that right?)

http://mvpbaseball.cc/stp.jpg
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by:Don Johnston
ID: 11981265
My understanding is that STP being an IEEE protocol, they decided on the IEEE version of Ethernet (802.3). As for why Cisco chose the SNAP frame for CDP... Not a clue. :-)

Regrading the errors on your BPDU's, yes, that is acceptable terminology. Is it just the BPDU's that are bad?

-Don
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Author Comment

by:dissolved
ID: 11983183
What are BPDUs? Ive been noticing a lot of chesk sum errors within all differnet tyeps of paclets. Dont know whats up with that.
thanks don
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Expert Comment

by:Don Johnston
ID: 11983916
BPDU's are Bridge Protocol Data Units. Those are what Spanning Tree Protocol (STP) frames are called. You mentioned STP problems and CRC errors and I assumed they were related.

If you're getting checksum/CRC/FCS errors on all your frames, the first thing I would check is if they're recent. Clear your counters and check again after 5, 10 and maybe 30 minutes. If you're still getting errors, is it on all interfaces? (probably not, but it doesn't hurt to ask). If not, it's probably a bad cable or connector... or noise.

-Don
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