Client Access Servers and Windows NLB?
Hi,I have following questions regarding NLB with CAS. My setup is 2 CAS servers running on Windows 2008 with NLB enabled.Which mode of NLB is recommended for E2k7 CAS - Unicast or Multicast? What are the issues when using multicast with Exchange functionality?I had setup NLB to work in unicast. In this mode I am able to reach cluster in the same subnet but not from another subnet. Is it required to make create/check registry entry mentioned in the "Masking the WLBS Cluster MAC Address" section of following article?http://support.microsoft.com/kb/193602I had setup NLB to work in multicast, it works fine from client subnet, but if using multicast follwoing are my concerns.it is not recommended for Exchange?It can cause port flooding, to avoid this I can use IGMP multicast, which has its own limitations/requirements like the one mentioned in follwoing link"You can only enable IGMP support when your cluster is configured for multicast mode. If you enable IGMP support, the permitted multicast IP address is restricted to the standard class D range. This is 224.0.0.0 to 239.255.255.255."http://support.microsoft.com/kb/323437Follwoin are two articles, one says, Unicast causes port flooding, another says, multicast causes port floodingLook for follwoing Q inhttp://technet.microsoft.com/en-us/library/cc783135.aspx Q. What is the Difference Between Multicast and Unicast Modes of Operation? Which One Should I Use in My NLB Installation?Multicast floodinghttp://technet.microsoft.com/en-us/library/cc781305.aspxPlease help me to choose best mode of NLB to be used with CAS.Any help/suggestions are highly appreciated. Thanks!
February 2nd, 2009 2:21am

Dear customer: 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: l Unicast mode induces switch flooding, where all switch ports are flooded with NLB traffic, even ports to which non-NLB servers are attached; l 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. It is hard to say which one is better for unicast and multicast. Also, for more information about them, please send the issue into the following forum: Microsoft TechNet > Forums Home > Windows Server > Platform Networking http://social.technet.microsoft.com/Forums/en-US/winserverPN/threads/ Rock Wang - MSFT
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February 2nd, 2009 6:06am

Hi Rock,Thanks for the reply.I have already gone through the phrase you have taken from folowing article,http://technet.microsoft.com/en-us/library/cc783135.aspxIt says that unicast induces switch flooding, but that applies to multicast as well as per follwoing link.http://technet.microsoft.com/en-us/library/cc781305.aspxYou can avoid it using IGMP, which has its own requirements in terms of networking, IGMP must be enabled on NW cards, switches etc. Also,"You can only enable IGMP support when your cluster is configured for multicast mode. If you enable IGMP support, the permitted multicast IP address is restricted to the standard class D range. This is 224.0.0.0 to 239.255.255.255."http://support.microsoft.com/kb/323437Also, as per my understanding following point is not correct. l 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. This is true only if you have single NIC card, its not a limitation when using 2 NIC cards. I will also send the issue to networking forum, however since this an Exchange forum, i am expecting to get the answerso following.Which mode of NLB is recommended for E2k7 CAS - Unicast or Multicast? What are the issues when using multicast with Exchange functionality?I had setup NLB to work in unicast. In this mode I am able to reach cluster in the same subnet but not from another subnet. Is it required to make create/check registry entry mentioned in the "Masking the WLBS Cluster MAC Address" section of following article?http://support.microsoft.com/kb/193602I had setup NLB to work in multicast, it works fine from client subnet.Thanks!
February 2nd, 2009 8:03pm

Dear customer: Multicast provides no advantage over unicast from the switches perspective. The increased multicast processing overhead for routers and switches may lead to slower performance. Carefully analyze the effect on your network when you uses multicast to avoid congesting other network devices. For more information about How to load-balance Exchange 2007 Client Access Servers (CAS) using Windows Network Load Balancing (WNLB) technology, please refer to the following article: How to load-balance Exchange 2007 Client Access Servers (CAS) using Windows Network Load Balancing (WNLB) technology http://www.msexchange.org/articles_tutorials/exchange-server-2007/high-availability-recovery/load-balancing-exchange-2007-client-access-servers-windows-network-technology-part1.html For more information about Single network adapter, please refer to the following article: Single network adapter http://technet.microsoft.com/en-us/library/cc776178.aspx Rock Wang - MSFT
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February 3rd, 2009 5:27am

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