Microsoft Network Load Balancing Multicast and Unicast operation modes
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Microsoft Network Load Balancing Multicast and Unicast operation modes

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Article ID: 309578

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Updated On: 04-21-2025

Products

VMware vCenter Server VMware vSphere ESXi

Issue/Introduction

This article provides information about Microsoft Network Load Balancing (NLB) on ESX hosts, as well information about the communication modes Multicast and Unicast.

Environment

VMware vSphere ESXi 6.x
VMware vSphere ESXi 7.x
VMware vSphere ESXi 8.x

Resolution

A Microsoft NLB cluster has two communication modes: Multicast and Unicast.
 
Note: The communication mode depends on the physical switch's feature set. Cisco switches have Multicast turned on by default. Some HP switches do not support Multicast.
 

Unicast mode
 

  • In Unicast mode, NLB reassigns the station MAC (media access control) address of the network adapter for which it is enabled and all cluster hosts are assigned the same MAC address.
  • Unicast mode induces switch flooding, where all switch ports are flooded with NLB traffic, even ports to which non-NLB servers are attached.
  • Since all hosts in the cluster have the same IP Address and the same MAC Address, there is no inter-host communication possible between the hosts configured in Unicast mode. A second NIC is therefore needed for other host communication.

Note: In Windows Server 2003 Service Pack 1 and later, a second NIC is not required. Single NIC Unicast is possible using the registry key UnicastInterhostCommSupport.

Note: Unicast NLB nodes cannot communicate over an NLB-enabled network adapter in Windows Server 2003. 

Multicast mode
 

  • In multicast mode, NLB assigns a layer-2 multicast address to the cluster adapter instead of changing the adapter’s station address.
  • Multicast allows inter-host communication because it adds a layer two multicast address to the cluster instead of changing it. This makes inter-host communication possible as the hosts retain their original unique MAC addresses and already have unique dedicated IP addresses.
  • However, in multicast mode, the ARP reply sent out by a host in the cluster in response to an ARP request, maps the cluster's Unicast IP Address to its multicast MAC Address. Such a mapping in an ARP reply is rejected by some routers, so administrators must add a static ARP entry in the router mapping the Cluster IP Address to its MAC Address.
  • See also: IGMP/MLD snooping when enabled makes Virtual IP in Microsoft NLB (Network Load Balancer) not reachable

NLB and Virtual IP

This diagram describes the basic concept of NLB and virtual IP:
 
 

Each server in a Load Balancing Cluster is configured with a virtual IP address. The virtual IP address is configured on all the servers that are participating in the load balancing 'cluster' (a loose term that is unrelated to the Microsoft Cluster Service). When a request is made on this virtual IP, a network driver on each of these machines intercepts the request for the IP address and re-routes the request to one of the machines in the Load Balancing Cluster based on rules that you can configure for each of the servers in the cluster.

Any IP-based service can be run off this service.

 

Additional Information

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