From: Howard C. Berkowitz (hcb@gettcomm.com)
Date: Sun Jul 18 2004 - 16:34:13 GMT-3
At 3:14 PM -0400 7/18/04, Ned wrote:
> Can anyone shed a little light on the difference between ethernet
>types(802.2, 802.3, Ethernet 2)
802.3 is the standard version of the Ethernet industry specification,
also called DIX. 802.2 headers go at the very beginning of the frame
header.
>and what is the minimum and maximum frame size.
It depends -- if, for example, you count the preamble. The basic
802.3 frame has two 6-hyte address fields, a 2-byte length field, and
a 4-byte frame check sequence, and up to 1500 bytes of payload
(sometimes more). Ethernet uses the length bytes for a payload
protocol type code.
When you add 802.1p QoS or 802.1q VLANs, they add four bytes to the
header, creating what is called a "baby giant", defined in 802.3ac.
While not IEEE approved, there are various industry standards for
"jumboframes" up to 9122 bytes. Cisco platforms vary in their
support for jumboframes, often varying down to the level of interface
and supervisor card, not just platform.
>Ihave read different information on various sites. Also, when
>referred to Ialways assume in relation to ethernet that when they
>say octet it means thesame as bytes like for eg. max ethernet frame
>is 1518 octets(?? is thatcorrect).
Yes.
>My thoughts:802.2 (Ethernet 2) has the type field802.3 has the
>length field**That's the general difference but can someone
>elaborate.
Yes. The reason for the length field is that it lets the transmitter
pad small payloads up to 64 bytes; the length field tells you how
many padding bytes can be ignored.
>For eg. DSAP,SSAP.
When the length field preempted the Ethertype field, there was still
a need to know what payload was being carried. 802.2 Type 1 (types 2
and 3 are obsolete) headers carry that information either directly in
the Source and Destination Service Access Point IDs, or, indicated by
a particular set of DSAP/SSAP values, more commonly in the five-byte
SNAP extension to 802.2.
>What ethernet type is commonly used and how to tell which one is
>beingused without a packet analyzer.
I guess I don't understand -- how do you see the frame without a
packet analyzer.
>Max User Data: 1460 BytesMax Frame Size: 1518 Bytes (1460 + 20( ip
>header) + 20 (tcp header) + 18Bytes for the Ethernet Header (this
>does not include the preamble and theStart of frame Delimiter)What
>would the payload be? (would it include the ethernet header or would
>itbe 1500 which is the 1460 + tcp header + ip header)I have asked
>quite a few questions !
> but I really want to make sure I have agood understanding of this
>and if I don't I would really like to. Thanks abunch.
I've crossposted my response to the general list, as, without meaning
to offend, these are pretty elementary questions for the CCIELAB
list. You might also want to ask these on the Associates list.
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