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Collision Domains and Cut-Through

Trying to better understand what will happen to collision domains using Cut-Through, Store and Forward, Fragment Free.

I believe that regardless of forwarding type the "collision domain" will stop at the switch port.

Collision domain defined as "if two stations TX at the same time their will be a collision"

So if using Cut-through, a frame is RX on port 1, the DA is looked at and it should be forwarded to port 2, port 2 will still perform CSMA/CD to TX the frame out port 2, the switch will not become a repeater and just send the frame out. YES/NO

So the collision domain is not extended, I suppose the error domain is extended because the hosts on the other port will have to deal with a errored frame and as one Cisco doc describes when the switch RX's all the frame and notices it is an error it will stop sending the frame out the egress port.

If there is a collision it is not caused by Cut-Through.

I have found a few references skirting the question but nothing precise, does anyone know if specific documents describing this issue?

Thanks

dcss00

1 REPLY

Re: Collision Domains and Cut-Through

Hi,

to the cut-through collision problem:

What happens in the case of collision on egress port? The switch has to stop transmitting the frame and

1) buffer the complete frame to be able to transmit it (so it has to buffer the whole frame anyway, loosing the advantage of cut-through no-buffering)

or

2) send a collision signal back to the ingress port for the originating station to resend the frame (the collision domain extends).

The fragment free concept uses the assumption that in a correctly built Ethernet network (not exceeding the maximum cable length) a collision should be detected during the first 64 bytes of the frame being sent. So you can release the buffer after transmitting 64 bytes successfully. But you still need to buffer the complete frame in the case of collision.

Finally, nowadays switches are 10/100 or even 10/100/1000 half/full duplex and the cut-through buffers/qeues handling process became so complicated that no vendor is using it anymore.

All switches are store-and-forward nowadays.

Regards,

Milan

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