Archived from groups: comp.dcom.lans.ethernet (
More info?)
In article <1122561924.852078.241070@o13g2000cwo.googlegroups.com>,
"anoop" <ghanwani@gmail.com> wrote:
> Rich Seifert wrote:
>
> > No device was ever discovered, either then or (to my knowledge) now,
> > that did not display acceptable behavior when presented with a 1522 byte
> > tagged frame vs. a 1518 byte (maximum untagged) frame.
>
> I don't know of any specific examples where this is not true which is
> really weird since switches/bridges built pre-802.1Q should have
> dropped
> anything over 1518 bytes.
>
Why should they drop such frames? It takes *extra effort* to examine the
frame length, compare it against some reference length, make a decision
to discard, and then perform the discard (e.g., adjusting buffer
pointers, updating linked lists, etc.) Bridges/switches are generally
designed to be "lean and mean," i.e., built for performance.
The reason why the length is not a problem is that 1518 bytes (or even
1526 bytes, a maximum-length untagged frame plus preamble) is not a
"round number." If you are building a buffer-management scheme, you
would not organize memory in 1518-byte chunks. Most folks used 2K-bytes
(a nice round number, 0x0800--very easy to align in hardware); if you
wanted somewhat better memory efficiency, you would use 1536-byte
buffers (a fairly round number, 0x0600--not quite as simple as 0x0800).
Thus, as long as the VLAN extensions did not push the maximum frame
beyond 1536 bytes, no practical implementation would be unable to handle
such frames. Even allowing for storage of the preamble (some
implementations do this), you get a 1528 byte upper limit.
Even if a switch is using dynamic buffers, the granularity will not be
less than 64 bytes, since the switch always needs at least that length
for a minimum frame. In order to accommodate a 1518-byte frame, the
switch will need at least 24 64-byte buffers, which is exactly the 1536
byte number discussed above.
As long as the frame doesn't overflow the switch buffer, it should work
fine. Since buffers are invariably 1528 bytes or more, VLAN-tagging does
not affect frame forwarding by legacy switches in any negative manner.
--
Rich Seifert Networks and Communications Consulting
21885 Bear Creek Way
(408) 395-5700 Los Gatos, CA 95033
(408) 228-0803 FAX
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