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Jorge Arredondo Dorantes

Jorge Arredondo Dorantes

backplane vs switch fabric. Same concept or different?

Non-blocking switch: one that is capable of transferring the aggregate traffic load from all ports while they are running at wire speed (the actual maximum transmission rate of the link).
Switch fabric: the internal interconnection architecture that supports connectivity among the various ports of the switch. Sometimes called the "backplane".
Switch fabric (or backplane) bandwidth must be equal to or greater than the aggregate bandwidth of all ports on the switch, for a switch to be called "non-blocking".
I'm certainly no expert on this, but here's one person's perspective (mine) on the history of these figures.
Manufacturers used to quote switch fabric bandwidth as the number of ports times the bandwidth of each port. Early on, Cisco switches were at a performance disadvantage compared to competitors' products: they switched in software, while the competition did it in hardware. Hardware throughput was much faster.
To counter customer's perceptions that the Cisco switches were slower, the Cisco marketing types hit on the idea that the max. bandwidth was the same in each direction, so they doubled their figures and published that. If it was in print, it must be true; many customers bought into the new "Cisco math".
Other vendors, not to be outdone, started doing the same thing and doubled their figures. Whatever; as long as you knew how they were reaching their numbers, you could compare them fairly.
Somewhere along the way, Cisco seems to have stopped doing the "Cisco math", reverting back to the earlier way of calculating. Maybe it's because they now have switches that CAN do all their switching in hardware.
I'm not slamming Cisco, because I love their products, and make a living installing and maintaining them; just the marketing confusion they've caused.
The PDF chart that you posted a link to has some misleading figures in it. Because some seem to be calculated with the old "new math" (i.e. x2), others with the new "old math" (i.e. not x2). So I don't think it provides fair comparisons, even among Cisco's own switches. Furthermore, in some cases I think it oversimplifies, grouping whole sets of some switches under one line (e.g. 3500 XL for the 3512, 3524, 3548, and 3508G) and giving the max. Gbps of the fastest one (3508G). So I don't put too much confidence in it.
Maybe it's about time Cisco explained their math, and put together a table that specified the exact pps, Gbps, and blocking/non-blocking status of each and every switch model. (HINT, HINT.)
Let's run the numbers for one of the switches you mentioned, see how it compares with the PDF.
2970G-24T has 24 10/100/1000 ports.
24 ports * 1 Gbps per port = 24 Gbps (new "old math")
which is what the PDF quotes; I would say it's non-blocking.
Its switching performance is
24 ports * 1,488,000 pps max. per Gig port = 35,712,000 pps
which is close enough to the 35,700,000 the PDF quotes; I would also say that it's non-blocking from this figure.
OK so far. Let's try another one that you mentioned.
2950-24 has 24 10/100 ports.
24 ports * 100 Mbps per port = 2400 Mbps or 2.4 Gbps(new "old math");
even doubling it (old "new math") only gets you 4.8 Gbps, and neither of these figures shows up in the PDF, so I couldn't guess whether this switch was non-blocking or blocking without digging elsewhere on Cisco's web site for more information.
Its switching performance is
24 ports * 148,800 pps max. per Fast Ethernet port = 3,571,200 pps
which isn't even close to the 6,600,000 the PDF quotes; so again, I'd have to dig deeper for more accurate information on this switch than what's in the PDF table.
Not sure where the PDF's figures came from for this switch. Maybe it's a typo.
Moving on, let's look at a switch almost the same as the last one, but with a "G" in its model number.
2950G-24 has 24 10/100 and two Gigabit Ethernet GBIC ports.
24 ports * 100 Mbps per FE port = 2.4 Gbps (new "old math"); plus
2 ports * 1 Gbps per GE port = 2.0 Gbps (new "old math"); equals
4.4 Gbps (new "old math"); x2 (for old "new math") equals 8.8 Gbps,
which is the same figure quoted for the 2950"no G"-24 switch, but not quite the 13.6 Gbps the PDF equates with this switch. Hmmm.....
Its switching performance is
24 ports * 148,800 pps max. per FE port = 3,571,200 pps; plus
2 ports * 1,488,000 pps max. per GE port = 2,976,000 pps; equals
6,547,200 pps,
which is pretty close to the 6,600,000 the PDF quotes for the 2950"no G"-24 switch, but substantially less than the 10,100,000 pps listed for this switch. Hmmm.....
Looks like maybe they put the right numbers next to the wrong switch.
Without showing all the gory arithmetic here, it turns out that those figures shown in the PDF for the 2950G-24 are accurate...for the 2950G-48!
My point being, this PDF file may be a useful reference for someone in marketing who may not understand all the technical details, but it's so full of inaccuracies that it borders on false advertising from a technical perspective.
And to answer your question about SFP, that is short for Small Format Pluggable, and refers to the new miniature Gigabit transceivers that some Cisco switches now support instead of GBICs. The SFP modules use LC connectors instead of SC connectors, and are a little smaller than GBICs, so you can fit them into smaller spaces than a GBIC (or maybe fit more of them into the same space as a GBIC). SFPs and GBICs can communicate over fiber, but they are not interchangeable in a switch: that is, you cannot put an SFP in a GBIC opening, or a GBIC into an SFP opening.
Oh, and that x2 stuff only applies to figures calculating bits per second on the switch fabric/backplane, not packets per second of switching throughput. (Technically, it's frames per second when you're talking Layer 2 switching, and packets per second when it's Layer 3 switching. Just more confusion from that PDF.)
A blocking switch is one that cannot handle the max. traffic load of all its interfaces across its switching fabric/backplane at the same time. The 3508G was such a switch.
Hope this hasn't added to the confusion...

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