If you look at RFC 2469 you can see that no matter how the bits are stored
in memory they go on the wire the same way. It is impossible to distinguish
a canonical formatted message from a non-canonical formatted message. Here
is the excerpt:
Canonical form (also known as "LSB format" and "Ethernet format") is
the name given to the format of a LAN adapter address as it should be
presented to the user according to the 802 LAN standard. It is best
defined as how the bit order of an adapter address on the LAN media
maps to the bit order of an adapter address in memory: The first bit
of each byte that appears on the LAN maps to the least significant
(i.e., right-most) bit of each byte in memory (the figure below
illustrates this). This puts the group address indicator (i.e., the
bit that defines whether an address is unicast or multicast) in the
least significant bit of the first byte. Ethernet and 802.3 hardware
behave consistently with this definition.
Unfortunately, Token Ring (and some FDDI) hardware does not behave
consistently with this definition; it maps the first bit of each byte
of the adapter address to the most significant (i.e., left-most) bit
of each byte in memory, which puts the group address indicator in the
most significant bit of the first byte. This mapping is variously
called "MSB format", "IBM format", "Token-Ring format", and "non-
canonical form". The figure below illustrates the difference between
canonical and non-canonical form using the canonical form address
12-34-56-78-9A-BC as an example:
In memory, 12 34 56 78 9A BC
canonical: 00010010 00110100 01010110 01111000 10011010 10111100
1st bit appearing on LAN (group address indicator)
|
On LAN: 01001000 00101100 01101010 00011110 01011001 00111101
In memory,
MSB format: 01001000 00101100 01101010 00011110 01011001 00111101
48 2C 6A 1E 59 3D
Notice that on the LAN they look exactly the same.
Neil
""Ejay Hire"" <[EMAIL PROTECTED]> wrote in message
[EMAIL PROTECTED]">news:[EMAIL PROTECTED]...
> Roosevelt Giles, CCIE All-in-one P210-212. In translational bridging, the
> Translation engine converts the format from Little-endian to big-endian.
> The multicast bit and the U/l bit are both part of the mac-address, and
are
> converted with the address, with no modification other than the Bit
> re-ordering.
>
> After Bit translation, the Frame length, Access Control, Routing
> Information... fields are added to the fram, and it is passed to the
Bridge
> module.
>
>
> ----Original Message Follows----
> From: "Neil Desai" <[EMAIL PROTECTED]>
> Reply-To: "Neil Desai" <[EMAIL PROTECTED]>
> To: [EMAIL PROTECTED]
> Subject: Layer 2 Addressing
> Date: Thu, 14 Sep 2000 16:00:56 -0400
>
> 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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