Multicast and Unicast Modes - MS Network Load Balancing (NLB) Operation Modes

Via VMware Knowledgebase >>

Products

VMware ESX

Product Versions

VMware ESX 3.0.x VMware ESX 3.5.x VMware ESXi 3.5.x Embedded VMware ESXi 3.5.x Installable

Component

GSS_Network

Purpose

<span class="Apple-style-span" style="border-collapse: separate; color: rgb(0, 0, 0); font-family: verdana; font-size: 12px; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px;"><pre><span style="font-size: 10pt; font-family: Arial;">This article focuses on Microsoft Network Load Balancing (NLB) on ESX servers, and understanding the different operation modes: Multicast and Uni-cas</span><br class="Apple-interchange-newline" /></pre></span>

Resolution

A Microsoft Network Load Balancing (NLB) cluster has two communication modes: Important Note: Depending on physical switch’s feature set choose a NLB configuration: Cisco switches have Multicast feature turned on by deafult. Some HP switches do not support Multicast. Unicast mode In its default Unicast mode of operation, this is achieved by NLB reassigning the station (MAC) address of the network adapter for which it is enabled and all cluster hosts are assigned the same MAC (media access control) 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. Therefore a second NIC is needed for other host communication. Sample Configuration for Network Load Balancing (NLB) UNICAST Mode Configuration. (1006778) 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 IPAddresses. However, in multicast mode, the ARP reply sent out by a host in the cluster, in response to an ARP request, maps the clusters Unicast IP Address to its multicast MAC Address. Such a mapping in an ARP reply is rejected by some routers and so administrator must add a static ARP entry in the router mapping the Cluster IP Address to its MAC Address. Sample Configuration for Network Load Balancing (NLB) MULTICAST Mode Configuration.(1006580) The diagram below describes the basic concept of NLB and virtual IP.     Each server in a Load Balancing Cluster is configured with a 'virtual' IP address. Virtual IP address is configured on all the servers that are participating in the load balancing 'cluster' (a loose term that's unrelated to the Microsoft Cluster Service). Whenever 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. See Also Sample Configuration - Network Load Balancing (NLB) Multicast mode over routed subnet - Cisco Switch Static ARP Configuration Sample Configuration - Network Load Balancing (NLB) Multicast Mode Configuration  Sample Configuration - Network Load Balancing (NLB) UNICAST Mode Configuration

VMware, ESX Multicast, Unicast, NLB, Network Load Balancing, virtual IP, Microsoft