Virtual switching between VMs

[Mitrixsen]

Hello, everyone.

I have a quick question. I understand that inside our virtualized environment, we can run a virtual switch between our VMs. Some hypervisors come with pre-built switches and we can also deploy our own Cisco products (I think vNexus was one of them?) inside the hypervisor and configure like we would on a real hardware switch.

My book states the following:

Multiple vSwitches can be created under a virtualized server, but network traffic
cannot flow directly from one vSwitch to another vSwitch within the same host, and the
vSwitches cannot share the same pNIC.

Credit: CCNP and CCIE Enterprise Core ENCOR 350-401 Official Cert Guide page 832

Why do these restrictions exist? Why cannot we connect say vSW1 to vSW2 and why cannot these switches share the same pNIC? 

Why is it all implemented this way? I understand that vSwitches might forward traffic a bit differently than normal switches?

Thank you
David

[M02@rt37]

David,

From my point of view, the architecture assume that each vSwitch acts independently, representing a separate L2 domain inside the hypervisor. If 2 vSwitches need to communicate, this typically happen through a Vm or a virtual router that has interfaces connected to both vSwitches, effectively routing or bridging trafic between them.

A key design principle is that vSwitches are optimized to ensure VM isolation and security within the virtual environment.

[MHM Cisco World]

Book mention one vendor VM and one type of virtual Switch 

There are other vendor and different types of virtual switch without this limitations 

https://www.techtarget.com/searchitoperations/definition/virtual-switch

MHM

[bigevilbeard]

Trying to keep this simple, a vswitch cannot connect directly to one another within the same host, as they are meant for vm to hardware network connection, not internal virtual networking. By doing this it is enforcing a correct traffic flow and help keep security boundaries. The second thing is tey cannot share the same physical network, this helps isolation and seperation between networks.  

If you keep these two things in mind, both the restrictions exist because a vswitch are just simplified network to our standard hardware/physical switches, and are optimizations for specific traffic patterns and hypervisor integration. If your requires are to have inter vswitch communication, the design is to route traffic through your physical network infrastructure, as teh diagram shows.

Hope this helps.