Thread (2 messages) flat view 2 messages, 2 authors, 2003-08-06

RE: TOE brain dump

From: David Schwartz <hidden>
Date: 2003-08-06 21:13:52
Also in: lkml

This statement is completely false.  Ethernet switches *do* read the
packet into memory before starting transmission.
	Some do. Some don't. Some are configurable.
This must be so,
because an Ethernet switch does not propagate runts, jabber frames, or
frames with an incorrect ethernet crc.
	If they use cut-through switching, they do. Some use adaptive switching,
which means they use cut-through switching but change to store and forward
if there are too many runts, jabber frames, bad CRCs, and so on.

	Obviously, you can't always do a cut-through. If the target port is busy,
cut-through is impossible. If the ports are different speeds, cut-through is
impossible. The Intel 510T switch for my home network does adaptive
switching with configurable error thresholds. In fact, it's even smarter
than that, with an intermediate mode that suppresses runts without doing a
full store and forward. See:
http://www.intel.com/support/express/switches/23188.htm
If the switch starts
transmission before it's received the last bit, it is provably
impossible for it to avoid propagating crc-failing-frames; ergo,
switches must have the entire packet on hand before starting
transmission.
	Except not all switches always avoid propogating bad frames.

	DS
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