With Cisco switches you can pick to use one of the three modes: Cut-through, store-and-forward and fragment free.

Not mentioned is why you would use one of the other or use the higher latency one over just using the lower latency one.

Also, with processing and ram getting faster, wouldn't latency practically be none existent anyway if you were to use store-and-forward for extra reliability?

  • 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 7 '17 at 21:48

This is a lot more complex than just a software-selectable forwarding option - and is true not only for Cisco but most other vendors.

First of all - store-and-forward vs cut-through generally isn't software selectable on most switches. It's potentially available if the speeds of ingress and egress ports are equivalent but as soon as multiple speeds are involved (say 10G -> 1G, for example) then store-and-forward isn't an option but rather a basic necessity.

The entire premise of cut-through is that the switch doesn't need to wait for the entire frame to arrive but rather as soon as the destination of the frame is determined (..within the first few dozen bytes received) the frame is immediately copied to the output port. This is to say that the output port is transmitting while the receive port is still receiving. In the case of a mismatch of speeds this literally isn't possible, as the bits arriving on a 1G port aren't clocked quickly enough to be pushed out at 10G (and, similarly, need to be buffered in the other direction).

In contrast, store-and-forward means that a given switch needs to receive the entire frame before it's transmitted. This necessarily means that each switch incurs enough time for the frame to be de-serialized, processed and re-serialized. It also means that packets with errors can be dropped and other kinds of processing can be achieved. There are actually some other wins in this kind of approach as far as prioritization and certain kinds of fancy forwarding are concerned.

I don't know fragment-free refers to. There isn't really a notion of fragmentation in L2 switching. I've heard of various mechanisms of memory management concerned with fragmentation management but it's not directly applicable to the traditional s-and-f vs cut-through discussion.

To your last question - the lowest-latency switches out there tend to run fairly light buffers and fully symmetrical bandwidth which allows for the box to have as little processing (read: latency) as possible. This is true for both the Ethernet world as well as Infiniband.

The thing is that maps pretty well to the spines of spine-and-leaf designs but starts to break down when applied to the traditional requirements of actual at-scale networks in the real-world (read: 2-3 orders of bandwidth, ranging from 100M/1G/10G/25G at the edge to 10G/40G/50G at aggregation and 40/50/100G at the core). Put another way - good for spines, bad for leaves, worse for traditional networks.

The performance of store-and-forward is absolutely improving and the truth is that some switches can actually operate in a hybrid mode but store-and-forward is still pretty much overwhelmingly common. Faster memory and advances in bus design have absolutely contributed to this improvement but it's important to note that many of these improvements asymptotic and are tied to improvements in IC fabrication scale and, ultimately, physics (thus the oft-promised push for silicon photonics).

  • Freagment-free was cisco terminology for a cut-through mode that waited until enough octets had been received to ensure that the frame would not fall victim to a later collision. In a properly-designed collision domain that IIRC is equivalent to the min size frame 64 octets. It's legacy, don't think about it. This isn't dictated by CPU/RAM since it's in h/w, more by market demand. When 1G became common, cut-through was no longer seen as advantageous, particularly since multi-speed was common. With demand for low-latency for trading and storage cut-through is now popular again at 10G.
    – marctxk
    Jan 17 '17 at 17:24

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.