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© Rakhesh Sasidharan

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vSphere Distributed Switches are Layer 2 devices (doh!)

This is a very basic post. Was trying to read up on NSX and before I could appreciate it I wanted to go down and explore how things are without NSX so I can better understand what NSX is trying to do. I wanted to put it down in writing as I spent some time on it, but there’s nothing new or grand here.

So. vSphere Distributed Switches (VDS). These are Layer 2 switches that exist on each ESX host and which contain port groups that you can connect VMs running on a host onto. In case it wasn’t obvious from the name “switch”, these are Layer 2. Which means that all the hosts connecting to a particular Distributed Switch must be on the same Layer 2. Once you create a Distributed Switch and add ESXi hosts and their physical NICs to it, you can create VMKernel ports for Management, vMotion, Fault Tolerance, etc but these VMKernel ports aren’t used by the port groups you create on the Distributed Switch. The port groups are just like Layer 2 switches – they communicate via broadcasting using the underlying physical NICs that are assigned to the Distributed Switch; but since there’s no IP address as such assigned to a port group there’s no routing involved. (This is an obvious point but I keep forgetting it).

For example say you have two ESX hosts – HostA and HostB – and these are on two separate physical networks (i.e. separated by a router). You create a new Distributed Switch comprising of a physical NIC each from each host. Then you make a port group on this switch and put VM-A on HostA and VM-B on HostB. When creating the Distributed Switch and adding physical NICs to it, VMware doesn’t care if the physical NICs aren’t in the same Layer 2 domain. It will happily add the NICs but when you try to send traffic from VM-A to VM-B it will fail. That’s because when VM-A tries to communicate with VM-B (let’s assume these two VMs know each others MAC address so there’s no need for ARP communication first), VM-A will send Ethernet frames to the Distributed Switch on HostA who will then broadcast it to the Layer 2 network its physical NIC assigned to the Distributed Switch is connected to. Since these broadcasted frames won’t reach the physical NIC of HostB the VM-B there never sees it, and so the two VMs cannot communicate with each other. 

So – keep in mind that all physical NICs connecting to the same Distributed Switch must be on the same Layer 2. If the underlying physical NICs are on separate Layer 3 networks, and these Layer 3 networks have connectivity to each other, it doesn’t matter – the VMs in the port groups will not be able to communicate. 

And this is where NSX comes in. Using the concept of VXLANs, NSX stretches a Layer 2 network across Layer 3. Basically it encapsulates the Layer 2 traffic within Layer 3 packets and gives the illusion of all VMs being on the same Layer 2 network – but this illusion is what Network Virtualization if all about, right? :) VXLAN is an overlay

VXLAN encapsulates Layer 2 frames in UDP packets. The VXLAN is like a tunnel to which all the hosts connecting to this VXLAN hook into. On each host there’s something called a Virtual Tunnel End Point (VTEP) which is the “thing” that actually hooks into the VXLAN. If a VXLAN is a Distributed Switch made up of physical NICs from the host, the VTEP is the VMKernel ports of this Distributed Switch that do the actual communication (like how vMotion traffic between two hosts happens via the VMKernel ports you assign for vMotion). In fact, during an NSX install you install three VIBs on the ESXi hosts – one of these enhances the existing Distributed Switch with VXLAN capabilities (the encapsulation stuff  I mentioned above). 

Once you have NSX you can create multiple Logical Switches. These are basically VXLAN switches that operate like Layer 2 switches but can actually stretch multiple Layer 3 networks. Logical Switches are overlay switches. ;o) Each Logical Switch corresponds to one VXLAN. 

ps. VXLAN is one of the cool features of NSX. The other cool features are the Distributed Logical Router (DLR) and the Distributed Firewall (DFW). I mentioned that a ESXi host has 3 VIBs installed as part of NSX, and that one of them is VXLAN functionality? Well the other two are DLR and DFW (god, so many acronyms!). Prior to DLR if an ESXi host had two VMs connected to different Distributed Switches, and if these two hosts wanted to talk to each other, the traffic would go down from one of the VMs, to the host, to the underlying physical network router, and back to the host and up to the VM on the other Distributed Switch. But with DLR, the ESXi hypervisor kernel can do Layer 3 routing too, so it will simply send traffic directly to the VM in the other Distributed Switch. 

Similarly, DFW just means each ESXi hypervisor can also apply firewall rules to the packets, so you don’t need one centralized firewall place any more. You simply create rules and push it out to the ESXi hosts and they can do firewalling between VMs. Everything is virtual! :)

pps. Some other jargon. East-West traffic means network traffic that’s usually within or between servers (ESXi hosts in our case). North-South traffic means any other network traffic – basically, traffic that goes out of this layer of ESXi hosts. With NSX you try and have more traffic East-West rather than North-South. 

vSphere Distributed Switches are Layer 2 devices (doh!) by rakhesh is licensed under a Creative Commons Attribution 4.0 International License.