Layer-2 physical switches have the ability to maintain a MAC address table, lookup table, or CAM table. The table maps MAC addresses to switch port numbers. If there is no corresponding entry for a frame's destination MAC address in the lookup table, the switch will flood the frame via every switch port, other than the source port. To reduce the occurrence of such flooding, the switch has a mechanism to learn the MAC addresses and maintain a mapping table. It does so by reading the source MAC address information from the frames that enter the switch. Now, when you cable the physical NICs of an ESXi host to the physical switch ports, the switch is expected to see the MAC addresses of a number of vNICs. The switch will only be able to add an entry into the lookup table if the VMs start to communicate. VMware ESXi, however, can proactively notify the physical switch of the virtual machine's MAC addresses so that its lookup table is up to date even before a VM begins to communicate. It achieves this by sending a gratuitous ARP (seen as a RARP frame by the switch) with vNIC's effective address as the source MAC of the RARP frame. The RARP will have the destination MAC address set to the broadcast address—FF:FF:FF:FF:FF:FF.

ESXi will send out an RARP under the following circumstances:

• When a virtual machine is powered-on: The vNIC on the VM has to be bound to a physical NIC. The NIC chosen would depend on the load balancing policy used. To learn more about the load balancing policies, read the load balancing and failover section. When vSwitch assigns a physical NIC to a vNIC, ESXi will need to send an RARP frame to enable the switch to update its lookup table with a new physical port number mapping for the vNIC's MAC address. This is necessary because every time a virtual machine is powered-on, it is not guaranteed the same vNIC to PNIC mapping.
• When a virtual machine is migrated from one host to another: The vNICs of the corresponding VM will now be mapped to a physical NIC of the destination host. Hence, it becomes a proactive necessity to let the physical switch update its lookup table with a new port number for the vNIC's MAC address.
• When a physical NIC fails over: The vNIC will be re-distributed among the available active adapters. Again, it becomes necessary for the physical switch to be notified of the new port numbers for the vNIC MAC addresses.
• LBT automatically rebinds the vNICs to different PNICs: Based on the physical NIC load saturation levels. Since the vNIC to PNIC assignment is bound to change, the physical switch will need to be notified of the new port numbers of the vNIC MAC addresses.