Using Multicast WNLB

Posted: May 17, 2010 in Technology
Tags: , , , , , , ,

If you’re like me and have already configured your 2010 Exchange CAS in a WNLB cluster then you must have chosen 1 of 3 choices: Unicast, Multicast or IGMP Multicast. At first I went with Unicast as it was the easiest one to setup but then I realized that I may not want Unicast due to the nature of how it operates. An excerpt of a VMware white paper on MS load balancing reads:

Unicast mode works seamlessly with all routers and Layer 2 switchs. However, this mode induces switch flooding, a condition in which all switch ports are flooded with Network Load Balancing traffic, event ports to which servers not involed in Network Load Balancing are attached.

As you can see that may not be something you want to do. So I switched it up to Multicast, but once I did that I was no longer able to hit the virtual IP address associated with my CAS cluster. In order to fix this you must add a static ARP entry in the router, mapping the cluster IP address to its MAC address:

User Access Verification
Enter configuration commands, one per line.  End with CNTL/Z.

Where 10.X.X.X is the virtual IP address of my WNLB and YYYY.YYYY.YYYY is the virtual MAC address created for me by WNLB. After I completed that command I was able to connect to Exchange.


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