ESX uses network bridges

by Andy Archer

VMware ESX Server uses bridged networking to provide virtual machines with Ethernet access to physical networks. This bridge is in fact any physical network cards under the control of ESXs VMkernel, the LAN drivers for which, all written by VMware of course, don't actually function as NIC drivers but as bridging drivers.

When LAN switches hit the streets it was the death knell for hubs and bridges. Hubs connected all the PCs together and bridges split the network up into less busy chunks. Today switches provide all this functionality, and have improved on it as well, all in one handy component. As a result bridging is not quite as well understood as it once was.

BRIDGES

Bridges worked at OSI Layer 2, connected to 2 physical network segments analysing Ethernet frames and MAC addresses. When the destination MAC address was on the other side of the bridge the bridge would copy the frame onto the other network, otherwise it would let it carry on to its destination. In this way networks could be segmented into less busy parts.

This functionality is provided by the VMkernel where one network is virtual, the virtual switch, and the other is the normal physical network.

VMware Workstation, VMware GSX Server and VMware Server also provide this functionality and use it to provide VMs with a peer connection to the same LAN as the host.  ESX Server provides the same functionality although it doesnt make it as obvious as the other products do.

MAC ADDRESSES

Consider the MAC addresses that would arise from an analysis of a segment connected to an ESX physical NIC with Virtual Machines attached. The MAC addresses would begin 00:0c:29 or 00:50:56 this is due to the fact that the LAN driver on the ESX NIC is in fact a bridge driver, it simply uses the NIC to bridge between 2 networks, the virtual and the physical. No wonder VMware write the drivers.

TOPOLOGY

When planning the topology involving ESX Virtual Machines remember that the NIC in the VM connects to a switch, connects to a bridge, connects to a physical switch. It then all works and we can understand how the bits hang together.