DAC cable between two NetGear switches

Please understand that virtually all DAC cables are designated and sold as SFP+ and 10Gb - and there are reasons.

There are rare species of 1G-DAC cables. These support only 1G, and seem to identify to the switches on both ends as generic 1G SFP resp. GBIC modules.

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scsirob wrote:

My current setup is two GS324T switches, connected with two short Ubiquity DAC cables, that works well.

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This works because this switch model does allow using (passive) DAC. While it's not officially supported, the manual does warn that the max length DAC workable is 5 meters.

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scsirob wrote:

I have previously tried a GS324T to an older GS724Tv4 and that would not work with the same DAC cables, no link.

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Correct again: Many switches, like the GS724Tv4 do not support DAC links.

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scsirob wrote:

For 10Gb I now got an XS712T and tried to use the DAC cables to connect to either the GS724T or the GS324T, no link.

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This won't make a 10Gb - because neither the GS724T nor the GS324T have SFP+ ports. Here we need to understand that -no- SFP, SFP+, QSFP, ... do something we're used on Ethernet copper links: There is no link auto-negotiation, neither end-to-end not say on the PHY link between the modules. Exceptions are only SFP and SFP+ Ethernet RJ45 modules. 

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scsirob wrote:

The DAC cable itself links at 10Gb when connecting the two XS712 ports together.

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Of course, this is intended to work and supported on SFP+ ports, also because bot ends are 10Gb. But again: Always check what the switch vendors are supporting!

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scsirob wrote:

The DAC cable itself links at 1Gb when connecting the two GS724T ports together.

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This does not proof - even if several threads in communities state it different - that DAC are generically usable on SFP ports.

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scsirob wrote:

This tells me the DAC cables are compatible with the SFP slots of all switches, but the switches themselves refuse to talk to eachother. What might be the problem, and are there any known good DAC cables that will work with the XS712 and one of the other switches?

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Afraid no.

This does just proof that certain switch combinations are workable with 10Gb SFP+ DAC on 1Gb SFP ports. The same applies for purpose designed 1Gb SFP "DAC" - these might work between the XS SFP+ and the GS SFP ports - very similar to real SFP-fiber modules and a dual-fiber patch cable. No-name SFP Gigabit multi-mode fiber modules and patch cables are so inexpensive that it does not make much sense to search the rare SFP-"DAC" type cables. 

Said that: Avoid "special" module variants like BiDi (single fiber) - this requires again dedicated support on the switches.