I had this question last week around vSAN 2-node direct connect and whether using a crossover cable is still required to be used or if a regular CAT6 cable (CAT 5E works as well) can be used. I knew the answer and figured this would be documented somewhere, but it doesn’t appear to be. To be honest, many websites when talking about the need for crossover cables are blatantly wrong. And yes, I also spotted some incorrect recommendations in VMware’s own documentation, so I requested those entries to be updated. Just to be clear, with vSAN 2-Node Direct Connect, or vMotion, or any other service for that matter, you can use a regular CAT6 cable, combined with 10GbE (or faster) NICs, this gives you great performance without the cost of a 10GbE (or faster) switch. I can’t recall having seen a NIC in the past 10 years that does not have Auto MDI/MDI-X implemented, even though it was an optional feature in the 1000Base-T standard. In other words, there’s no need to buy a crossover cable, or make one, just use a regular cable.
Duncan, AFAIK there is no such thing as a crossover cable need for gigabit or ten gigabit.
The phy of those use all 4 pairs in full duplex, there is no TX/RX pair you can “cross”. The standard needs all pairs to be identified at each end…
We still have courses talking about half duplex and full duplex too. It’s just magic sauce, it seems.
Duncan Epping says
That is what the post discusses?
Ok, but there is still such a thing as a crossover cable, for typical ports are not just 1000BaseT but also support compatibility with legacy standards – E.g. the ports are usually 10/100/1000 BaseT, not just Gigabit. And there is still theoretically legacy support for half-duplex in the standard. So MDIX still exists…. It is just in practice as described in the original post.. every Gigabit port has the Auto MDI/MDI-X support.
Auto MDI/MDI-X maybe unnecessary when both sides are Gigabit and auto-negotiating, but it is necessary, and Auto MDI/MDI-X will not work in the event that port at one end of the cable or the other is configured to a manual or forced Speed and Duplex, e.g. Hard set to 100 Megabits-FD the other end will not be able to auto-negotiate not only Speed and Duplex, but also MDI/MDI-X cannot be negotiated, even if they are Gigabit ports.
For All SSD vSAN isn’t 10GB a requirement? if so and you don’t want to buy 10GB switch won’t you need a 10GB connection, if standard network is Gb then won’t you need a “crossover” as in you don’t need a cross over cable but you need to connect the two servers over 10GB to pass the checks?
Duncan Epping says
Correct, 10GbE is a requirement. This is why “direct connect” is perfect indeed, it avoids the need for a 10GbE switch, while still having the performance!
Hi Duncan. Is it possible to use twinax DAC cables for the direct connect network? I cannot find any documentation to state that this is or isn’t possible.