Hi,
That means NLB Cluster is configured as mutlicast and not unicast? NLB Cluster default operation is unicast, there might be some good reason why it was configured as multicast.
Excerp from Microsoft TechNet
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Q. What is the Difference Between Multicast and Unicast Modes of Operation? Which One Should I Use in My NLB Installation?
A. NLB relies on the fact that incoming packets are directed to all cluster hosts and passed up to the NLB driver for filtering. 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. In multicast mode, NLB assigns a layer-2 multicast address to the cluster adapter instead of changing the adapters station address.
Both modes of operation have their pros and cons. The advantages of unicast mode are that it works seamlessly with all routers and layer-2 switches (and layer-3 switches which are configured to operate in layer-2 mode). The disadvantages are:
• 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.
Multicast allows inter-host communication because it adds a layer two multicast address to the cluster instead of changing it and 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 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
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Regards,
Dandy
