1) I suppose that depends on the manufacturer. On most equipment placing
a striaght through cable on an MDI-X port makes it a crossover at the
port. That is the port is wired to cross 1,2,3 and 6 (hence the port
name MDI-X) so that you don't have to do it by changing the cable. So it
is not true that you MUST use a crossover cable per se, as the MDI-X
port does the crossing for you.
2) Hubs do not care about and have nothing to do with layer 2 at all.
Only layer 1. That being the physical connection. They simply repeat the
signals they recieve on one port out all other ports. Hubs do not know
the rule of CSMA/CD, they are manufactured to comply with the
requirements for physical connectivity. When a hub detects a collision
it does not respond to it except to sense that there was a voltage on
these pairs so I will make this little light turn red everytime I sense
voltage on these pairs. If a hub knew and participated in CSMA/CD, it
would backoff from transmitting everytime it sensed a collision and
before it ever repeated any data it would have to "listen" to the wire
before transmitting which it does not.
-----Original Message-----
From: Bradley J. Wilson [mailto:[EMAIL PROTECTED]]
Sent: Thursday, December 07, 2000 3:17 PM
To: cisco
Subject: Hub-to-Switch connectivity issue
Okay...I've been digesting the responses I've received about this issue,
and
what I've come up with can be reduced to two comments:
1) The fact that flipping the MDI/MDI-X switch didn't initially work
leads
me to believe that I wasn't using it right. The fact remains that
connecting a hub to a switch must be accomplished using a crossover
cable.
2) Why is that? It's true that connecting devices at different layers
of
the OSI model requires a straight-through cable, and that connecting
devices
at the same layer requires a crossover cable. So isn't it true that
switches are layer 2, and hubs are layer 1? No. Hubs are actually
layer 2
devices, and here's why: while a hub may not understand or care about
source
and destination MAC addresses, checksums, or what's in the "type" field,
it
*does* know the rules of CSMACD communication, and has to play by the
same
rules as other layer 2 devices. Therefore, hubs can be considered Layer
2
devices, and thus must be connected to switches with a crossover cable.
Anyone see anything wrong with my synopsis above? Let me know. Just be
careful with your caps lock key. ;-)
Thanks,
BJ
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