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At one point, we had a system which performed best when is 100/MB half duplex mode. (purely anecdotal evidence). It was plugged into a switch with other systems running at 100 Mbs full. Seemed ok and NW guys did not report any issues.

But how can you have systems on the same switch running different duplexes like that? Won't the 100Mbs half systems basically try to use CSMA/CD and 'disrupt' the other attached systems? I did not get a very good answer from the NW person at them time.

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In a hub, all the ports are electrically connected, and only one device at a time can talk on the wire. If more then one device talks at the same time, it creates a collision, so hub ports are all half-duplex.

In a switch, each port is isolated, and traffic from one port is selectively sent to other ports, so there are no collisions in the switch. Unlike a hub, where all the ports together are in one collision domain, each switch port is a separate collision domain. That means that each switch port can be set to full-duplex (when connected to a single device supporting full-duplex) or half-duplex (when connected to a hub or other device requiring half-duplex).

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  • I see. "Each is a separate collision domain" on a switch. That's how. – jouell Nov 10 '15 at 4:29
  • This answer is correct except for the first sentence. – Peter Green Oct 11 '18 at 1:41
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The mode/speed is configured on the switch port!
So you could have 12 ports in half-duplex(for devices that don't support full-duplex) and 12 ports in full-duplex. On the assumption that you're on a physical "star topology", those in "half" won't disrupt the others.

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  • OK then what exactly is it that a star topology would make this work? I understand that the mode is configured at the switch port level. How would multiple duplexes exist on a switch without error? Doesn't CSMA/CD mess it up? – jouell Nov 7 '15 at 18:05
  • On this little picture, it's a star topology. By design the SWITCH makes sure those lines never cross, so there's no collisions. With full-duplex, each single line becomes a highway with no crossroads. That's one of the many things a SWITCH does. Collision-domains – Seb B. Nov 7 '15 at 19:00

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