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I understand the principle of collision domains and how they are issues in hubs. I also get how switches are actually creating its own collision domain on each port. Therefore Port to Port communication creates a collision domain and takes care of collisions. It is all find and dandy here. What about the situation when I have two switches -- let's assume 48 ports each, daisy-chained together? What will happen when 48 ports on switch1 will receive transmissions at the same time and need to resend it to 48 ports on switch2? I assume that all 48 signals should be transferred over the SINGLE daisy-chain link. Will that one link introduce ONE collision domain for all 48 ports? I logically assume that it won't, but I don't understand how.

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    Typically, modern PCs and switches use full duplex connections, so there are no collisions. You seem to allude to a bandwith overload on a switch uplink, and that is a completely different discussion. – Ron Maupin May 8 '15 at 18:11
  • Yes, full duplex on single collision domain (port to port) therefore it is sending and receiving on different wires in the jack. But daisy chain groups all ports into one link, right? So how the layer one bits are not colliding when signal is moved from switch to switch? – Andrew May 8 '15 at 18:37
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    Properly configured and connected full duplex connections mean that there are NO collisions. You seem to mean a bandwidth overload, and that is a completely different question. – Ron Maupin May 8 '15 at 20:23
  • Did any answer help you? if so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer. – Ron Maupin Aug 9 '17 at 23:06
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If all 48 ports attempt to TX to the same "port" at the same time, it will most likely overrun the internal buffer space and "no buffer" (in cisco circles) drop the frame(s). As this is all at layer-2, no error is communicated to anyone; the frames just don't get there.

(This is all part of the store-and-forward processing of switches.)

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Back all the way up to the premise of the question. A collision, as it were, can only happen in a half duplex situation when "the wire" is common to both receive and transmit, and both sides try to transmit at once making the output from the wire unusable. So, even between two PCs attached to the same switch that communicate with each other, the two ports are not a collision domain as long as both hosts are linked at full duplex. If two hosts do happen to be both half duplex, then there is a collision domain between the switch and the host, which will be arbitrated by the efficacy of the carrier sense logic of each.

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Frames are passed over Ethernet. A single broadcast frame is all that is required to tell switch 2 to propagate the broadcast to all of its 48 access ports, not 48 individual frames.

Is that what you were looking for?

  • He wasn't talking about broadcast frames. – Ricky Beam May 8 '15 at 18:50
  • Right << What will happen when 48 ports on switch1 will receive transmissions at the same time and need to resend it to 48 ports on switch2? >>. In this case the uplink port between the source switch and the destination switch will transmit all 48 frames at near the speed of light. Virtually instantly. Right? – Ron Royston May 8 '15 at 22:47
  • S1 Port A sends to a port on S2, repeat for every port. He said NOTHING about broadcast. This is the very situation for which buffers and queues exist. Many switches do have the buffer space to handle this, but it's close to the limit. In the case of broadcast, it's much simpler as it will just be a single frame copied to every port instead of a full 48x48 matrix pushed into a single channel. – Ricky Beam May 9 '15 at 2:41

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