Exchange 2007, VMware, NLB, Cisco Catalyst and ARP

Recently I worked with a customer to implement a vmware based Exchange 2007 setup with two NLB-clustered CAS. We followed VMware’s recommendations regarding uni/multicast settings and choosed multicast for the NLB cluster (http://www.vmware.com/files/pdf/implmenting_ms_network_load_balancing.pdf). We realised after some testing that we could not reach the cluster IP from outside the local server subnet where the NLB nodes where sitting.
 
After some digging we found that there is a couple of things to consider when using multicast and Cisco equipment. Im not an network specialist so Im not going to give you a complete description of the problem. But in short the Cisco devices do not accept an arp reply for a unicast IP address that contains a multicast MAC address. In multicast mode the virtual IP address therefore becomes unreachable when accessed from outside the local subnet. To solve the problem we followed this guide from Cisco: http://www.cisco.com/application/pdf/paws/107995/microsoft_nlb.pdf and enabled IGMP Multicast for the NLB cluster.
 
 
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