Optics, and why they do not always fit
A transceiver is a module a few centimetres long that half of all network faults at handover can be traced back to. Not because it is broken, but because it is rejected, or because the type does not match the distance. This page sets out what there is to choose and where it goes wrong.
Five types, and where they belong
The distinction is about speed and distance, and together those two decide everything.
- SFPThe classic module up to a gigabit, in copper or fibre. Still the bulk of what goes into access switches, and ample for a floor or a wing of a building.
- SFP+ and SFP28Ten and twenty-five gigabit, the standard for uplinks and for the link between switch and server. This is the category most orders land in.
- QSFP+ and QSFP28Forty and one hundred gigabit, for the core of the network and between racks. They can often be split into four slower links, which is a cheap move when expanding.
- DAC and AOCA cable with the modules attached. Within one rack a DAC is cheaper, uses less power and is more reliable than two separate modules with a patch cable between them; beyond about seven metres it is an AOC or loose fibre.
- Short and long reachSame speed, different optics: the short variant covers hundreds of metres on multimode, the long one tens of kilometres on singlemode. They only work if both ends are the same type, and that is the mistake made most often.
Three reasons a module does not work
In none of the three is the module broken, and in all three the port stays dark.
The switch rejects it
Manufacturers program a code into their modules and part of the equipment accepts only that code. A third-party module should therefore be supplied coded for the brand it goes into, and then it simply works.
The fibre does not match the module
Multimode optics on singlemode fibre gives no link, or a link that drops out now and then. The second is worse, because then people spend days looking at something else.
The distance falls outside the range
Too far is a familiar problem; too close is not. A long-reach module over thirty metres can overload the receiver, and then an attenuator is needed rather than a different module.
Original or third party
There is no general answer; there are three situations with an answer each.
- Where third party makes senseLarge numbers of identical modules in access and distribution layers. The optics are a standard, the price gap with the branded module is considerable, and supplied coded it works like the original.
- Where you take the originalEquipment under a running vendor contract where support could become a discussion, and links where you do not want a second variable during an outage.
- What you record either wayWhich modules sit where and of what type. Without that list, the next expansion costs an afternoon working out what fits, and it is the same afternoon every time.
Coded for these brands
A third-party module should be supplied coded for the device it goes into. These are the codings most often asked for.
- Cisco and JuniperThe two where the check on the code is strictest and where an uncoded module is therefore rejected most often. Report the switch software version with the request as well.
- Arista, HPE and HPE ArubaHere the code is usually softer, but the neighbouring issue is firmware: an older version does not know newer modules.
- Extreme Networks, Brocade and NETGEARBrocade stands apart on the SAN side: there the Fibre Channel generation weighs more heavily than the coding.
- Servers and storageDell, IBM, Lenovo, Supermicro, NetApp and Fortinet. The network card in a server has its own list of approved modules, and it is not the same as the switch beside it.
What goes with this
Optics hang on the network around them.
Questions we get about this
The ones that come up most, answered briefly.
Does a third-party module void my warranty?
In practice the discussion is rarely the warranty on the switch and usually the support: during an outage a manufacturer may ask you to swap in its own module before looking further. Where you do not want to run that risk we advise the original module, and that is a choice per link and not per network.
Can you code a module for my brand?
Yes, that is the usual way of supplying it: the module is coded for the brand and type it goes into, after which it behaves like the original. So give us the brand, the type and the software version of the switch with the request.
How do I know which module suits my fibre?
It follows from the fibre that is in place and the distance to be bridged. Multimode asks for different optics than singlemode, and too close is just as much a problem as too far: over a short distance a long-reach module can overload the receiver and an attenuator is needed.
The right module, first time
Give the brand and type of the switch, the distance and the fibre that is in place. You get back which module belongs with it and whether a DAC is cheaper and better in this case.
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