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Looking here for an example, I see:

Spine Device Interface Summary As previously stated, the validated design includes up to 4 spine devices and up to 96 leaf devices that are interconnected by one or two high-speed Ethernet interfaces. A spine device must support 192 high-speed Ethernet interfaces to connect to 96 leaf devices using 2-member aggregated ethernet interfaces.

QFX10008 and QFX10016 switches were used as they can achieve the 192 ports necessary for this reference design. See QFX10008 Hardware Overview or QFX10016 Hardware Overview for information on supported line cards and the number of high-speed Ethernet interfaces supported on these switches.

I'm wondering if you need, at some point, physical switches to connect the cables to servers (even though you have virtual switches), or if there is something else in between cable and server.

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Of course, physical hardware is required to interface between the physical and the virtual world, but not necessarily switches.

A virtual host with just a single NIC could house an entire large virtual network with its own virtual switches and routers. Virtual machines and switches are connected by logical connections (associations), not cables. Cables are used in the physical world.

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  • Hmm, not quite what I'm wondering. I'm wondering, say you have 1000000 physical servers. How you go from the cables (somewhere that enter the building) to those servers, if those servers themselves are to (at least some of them) host the virtualized network. Wondering if you're suggesting you need two networks, the physical one for actually connecting the physical servers (in which case you need physical switches, there's no alternative), and the virtual one lives inside that. – Lance Pollard Jan 2 '19 at 8:58
  • I was sort of imagining just having a virtual network on some servers, and the servers would be somehow directly connected to each other. Then the input cable would go to one of the servers to start, and it would act as the router/switch. So I was wondering how the cables need to be organized/connected. But hmm, it seems like it might just be that you need two layers, a physical and a virtual. Not sure yet. – Lance Pollard Jan 2 '19 at 9:01
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    Multiple virtual hosts are most often connected by Ethernet as well, so a frame in between changes from virtual to physical and back again. Note that practically all networking (OSI) layers can be virtual or physical - we only deal with L1-L4 here, however. – Zac67 Jan 2 '19 at 9:08

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