Cut-through frame transission mode !!!

Dowima

If i have enabled a cut-through on a switch then connect it to the network !!! How the switch will learn addresses of source frames if cut-through is used ???
the switch suppose to read only the destination address and it is in the first 6 bytes.

jediknight

The same way that a switch would normally learn addresses... by reading the source MAC Address and adding it to the MAC Address Table if it is not known.

The switching forwarding modes Cut Through, Fragment Free, Store and Forward do not change how the switch learns addresses. The goal is to reduce latency by not having to store the entire frame in the memory buffer before forwarding - hence Cut Through and Fragment Free, but honestly with todays switches Store and Forward is pretty much as fast as Cut Through.

r_durant

I could be wrong here, but i dont think the switch "only" reads the destination address, I think it will read "up to" the destination address, but would have read and added the source address to it's MAC table before it gets to the destination address...this is just my understanding

Otherwise, I dont think a switch could be a switch without using MAC addresses...that would make it a hub...

But, i'd wait for someone to confirm or deny my claim... ...it would help me out as well, good question...

Dowima

jediknight That's why i'm confused and let us assume that there is a big difference in latency when using store-and-forword compared with cut-thru. Why in books they say it only reads destination mac.

So is that just a theory ? if not then how did they let the switch learn addresses?

Netstudent

Very good question.

dtlokee

It still looks at the source address regardless of the switching mode, the difference with cut through is that it will make the forwarding decision as soon as it receives the destination address in the frame.

r_durant

Question though...how do you enable cut-through, fragment-free or store-and-forward on a switch?

Dowima

Never thought of that
Anyone knows

dtlokee

r_durant wrote:

Question though...how do you enable cut-through, fragment-free or store-and-forward on a switch?

Depends on the model, some you can't, the higher end switches seem to support these configurations where the fixed switch configurations (like the 2950) support modified cut-through, it uses cut through unless there is a high rate of errors on the link or switch then it falls back to using store-and-forward. What is a high rate? I don't know, I have never seen it documented.