I have asked this question once before and had little luck in finding the
answers. This time I have given a more explination and an example.
I have a problem in understanding the issues concerning canonical vs.
non-canonical addressing. I have read the archives of GroupStudy.com (both
CCIE and regular mailing lists), RFC 2469, Optimized.com, Interconnections
(Second Edition), many Cisco Press Books. I have been to many college sites
in reference to Manchester encoding. I have searched on CCO, and the
Internet. I have had discussions with my peers and have now thoroughly
confused them. In the scheme of things I guess it does not matter on why but
that it just happens and that we need to be aware of the issues and how to
solve them. Unfortunately I can't leave it at that.
In "Interconnections Second Edition" pages 32-33 she states:
"With 802.3 and 802.4, the least significant bit is transmitted first; with
802.5 (and FDDI), the most significant bit is transmitted first. This would
not be an issue (adapters on the receiver and transmitter for a particular
LAN would presumably be symmetric, and the order of the transmission would
be irrelevant) except that the group bit in addresses was defined not as
"the most significant bit" or "the lease significant bit" but rather as "the
first bit on the wire." Thus, an address that was a group address on 802.3
would not necessarily look like a group address when transmitted on 802.5
because a different bit would be transmitted first.
The canonical format of address assumes least-significant-bit-order-first
order. Therefore, the address a2-41-59-31-51 is not a group address because
the least significant bit of the first octet (a2, which equals 10100010
binary) is 0.
When address are stored for transmission onto 802.5 or FDDI, which transmit
the most significant bit first, they must be stored in a different format.
Figure 2.9 shows the address a2-41-42-59-31-51 as stored for transmission
least significant bit first.
10100010 01000001 01000010 01011001 00110001 01010001
Figure 2.9 Address a2-41-42-59-31-51, least significant bit first
Figure 2.10 show the address a2-41-42-59-31-51 as stored for transmission
most significant bit first.
01000101 10000010 01000010 10011010 10001100 10001010
Figure 2.10 Address a2-41-42-59-31-51, most significant bit first
Therefore, bridges must shuffle the address fields when forwarding between
802.5 (or FDDI) and any other LANs."
>From all of the reading this is what I think to be true. If I am wrong in my
assumptions please let me know.
1. When an adapter needs to set the MAC address of a packet it will put it
in whatever format that it is accustomed to and is unaware of any other
format.
2. Regardless of how the packet is stored in memory it will transmit the
Global bit first. This is what a transmittion would look like:
Packet in canonical format: A8 is the Global bit, least significant bit
A1A2A3A4A5A6A7A8 B1B2B3B4B5B6B7B8 C1C2C3C4C5C6C7C8
Packet in non-canonical format: A8 is the Global bit, most significant bit
A8A7A6A5A4A3A2A1 B8B7B6B5B4B3B2B1 C8C7C6C5C4C3C2C1
Either way is transmitter with the Global bit first so both should look like
this on the wire:
A8A7A6A5A4A3A2A1 B8B7B6B5B4B3B2B1 C8C7C6C5C4C3C2C1
When an adapter receives a packet it should automatically rearrange the
packet into the appropriate format and everything should be fine.
I know how to do the conversion and when I do the conversion I can see the
problem. When I go through the steps of how a packet is formed I can't see
the problem.
I would appreciate a reply, answer or direction to go from anyone.
Thanks,
Neil
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