SFP compatibility, LX ethernet and FC SFPs, and other noob questions

tinfoil3d

Member
May 11, 2020
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Hello.

I'm looking to try out the optical networking(I foresee a need to connect two buildings several hundred meters apart with fiber).
I've never touched SFPs at all.
Some questions before I spend my dogecoins on network gear.
1. Do I get this right that FC SFPs (2,4,8,16 etc) are INCOMPATIBLE with ethernet-type adaptors/switches? Because I see them in large quantities on ebay for much, much lower prices than *GBase-LX SFPs. I just wanted to buy some transcievers(1 gig) for experimenting and some used switches and adapters just to play around with but it's not gonna work as ethernet?
2. Suppose I have QSFP-something faster mellanox or something with speeds of 40-50GB/s ethernet. Can I connect this faster adapter with a fiber into a much slower, 10GB/s or even 1GB/s transciever on the other end(assuming both sides are mode-compatible and lw length compatible)? For instance, mellanox docs specifically state speeds of 50,40,25,10,1. Do I understand it right that it will auto-negotiate with the other side? Basically, QSFP-something connected to SFP/SFP+ with a fiber. Will that work? In general?

A little bit off-topic, is FC SAN alive at all? I know it's alive like 5.25 HDDs, but does anyone here see NEW installations of it in production environments? Can't you get a much better performance with just about any other tech nowadays?
 

Jab.R

New Member
Apr 16, 2016
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FC SFP adapters are likely to work at 1Gbe. You need to try.

QSFP+ single mode (LR) will connect to SFP+ singlemode or SFP28 singlemode. (LR) when the QSFP+ module supports 10 and 25 respectively
 

gregsachs

Active Member
Aug 14, 2018
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FC SFP adapters are likely to work at 1Gbe. You need to try.

QSFP+ single mode (LR) will connect to SFP+ singlemode or SFP28 singlemode. (LR) when the QSFP+ module supports 10 and 25 respectively
Can also usually use a qsfp->sfp adapter and just use 1/4 of the 40gbe link.
 

tinfoil3d

Member
May 11, 2020
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Thanks, I just thought that FC SFPs are like all SFPs, not exactly level 0, because there is protocol(speed) negotiation between them. So I thought FC SFPs talk on a completely different stack. Or is it really level 0 and negotiation is done by the "port" in equipment accepting SFPs?
 

Blinky 42

Active Member
Aug 6, 2015
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Considering new 1G LX SFP's are < $10 I would just get new ones where you don't have to worry about the emitter getting out of spec because it is old as hell or clogged up with gunk because some random kid ripped them out of a datacenter and tossed them in a bag.
If you expect to need to connect buildings with new fiber, run single-mode and just get 1310nm LX modules to start so you have everything but the run between buildings ready when you do install it.

Yes some FC modules will work in 1G with multimode, but I wouldn't spend any $ on buying them. It is more the level of experimenting because they are are still in the FC switch at the bottom of the junk pile or someone gave them to you with a few miles of old orange OM1 patch cables :)
 
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tinfoil3d

Member
May 11, 2020
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Thanks for suggestion, blinky 42. I already got an idea on modes and know that SM is more expensive but can go kilometers on a very high speeds.
 

psc

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Jun 30, 2019
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Thanks for suggestion, blinky 42. I already got an idea on modes and know that SM is more expensive but can go kilometers on a very high speeds.
It has that reputation, and for new kit the optics are more expensive, but I've previously picked up a job lot of 10 brocade 10G SM transceivers for peanuts. I take the point from Blinky 42 about 2nd hand, but take some comfort from having signal quality metrics available (and still having spares) and the fact I'm not running a production site. If you're talking several hundred meters SM seems the way to go.