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General Considerations

Applies To: Windows Server 2003 with SP1

  • Some routers require a static ARP entry because they do not support the resolution of unicast IP addresses to multicast media access control addresses. For example, Cisco routers require an ARP (address resolution protocol) entry for every virtual IP address. While Network Load Balancing uses Level 2 Multicast for the delivery of packets, Cisco's interpretation of the RFCs is that Multicast is for IP Multicast. So, when the router doesn't see a Multicast IP address, it does not automatically create an ARP entry, and one has to manually have to add it on the router.

  • Network Load Balancing can operate in two modes: unicast and multicast. Unicast support is enabled by default, which ensures that it operates properly with all routers. You might elect to enable multicast mode so that a second network adapter is not required for communications within the cluster. If Network Load Balancing clients access a cluster (configured for multicast mode) through a router, be sure that the router accepts an Address Resolution Protocol (ARP) reply for the cluster's (unicast) IP addresses with a multicast media access control address in the payload of the ARP structure. ARP is a TCP/IP protocol that uses limited broadcast to the local network to resolve a logically assigned IP address. Verify that all cluster hosts are operating in unicast or multicast mode, one or the other, but not both.

  • If the cluster is operating in unicast mode (default setting), Network Load Balancing cannot distinguish between single adapters on each host. Therefore, any communication among cluster hosts is not possible unless each cluster host has at least two network adapters.

  • You can configure Network Load Balancing on more than one network adapter. However, if you do bind NLB to a second network adapter ensure that you are configuring them correctly.

  • Use only the TCP/IP network protocol on the adapter NLB is enabled for. Do not add any other protocols (for example, IPX) to this adapter.

  • Enable Network Load Balancing Manager logging. You can configure Network Load Balancing manager to log each Network Load Balancing Manager event. This log can be very useful in troubleshooting problems or errors when using Network Load Balancing Manager. Enable Network Load Balancing Manager logging by clicking Log Settings in the Network Load Balancing Manager Options menu. Check the Enable logging box and specify a name and location for the log file.

  • Verify that the following is true for cluster parameters, port rules, and host parameters:

    • Cluster parameters and port rules are set identically on all cluster hosts.

    • Port rules are set for all ports used by the load-balanced application. For example, FTP uses port 20, port 21, and ports 102465535).

    • Always click Add after setting a port rule. Otherwise, the port rule will not appear in the list of rules, and the rule will not take effect.

  • Ensure that the dedicated IP address is unique and the cluster IP address is added to each cluster host.

  • Verify that any given load-balanced application is started on all cluster hosts on which the application is installed. Network Load Balancing is not aware higher level applications and does not start or stop applications.

  • Verify that the following is true for the dedicated IP address and the cluster IP address:

    • Except in the case of a virtual private network (VPN), both the dedicated IP address and the cluster IP address must be entered during setup in the Network Load Balancing Properties dialog box and also in the Internet Protocol (TCP/IP) Properties dialog box. Make sure that the addresses are the same in both places.

    • When configuring a VPN load balancing cluster, you should not configure the dedicated IP address. On a VPN, only the cluster IP address should be present on each of the cluster hosts because clients running Windows 95, Windows 98, or Windows NT 4.0 may be unable to connect to the cluster if the dedicated IP address is configured on the Network Load Balancing cluster hosts. If you omit this step, the cluster will converge and appear to be working properly, but the cluster host will not accept and handle cluster traffic.

    • Ensure that the dedicated IP address is always listed first (before the cluster IP address) in the Internet Protocol (TCP/IP) Properties dialog box. This will ensure that responses to connections originating from a host will return to the same host.

    • Both the dedicated IP address and the cluster IP address must be static IP addresses. They cannot be DHCP addresses.

    • Ensure that all hosts in a cluster belong to the same subnet and that the cluster's clients are able to access this subnet.

  • No special cluster interconnect is used by Network Load Balancing. NLB uses the same network interface to maintain cluster state awareness.

  • Do not enable Network Load Balancing on a computer that is part of a server-cluster cluster. Microsoft does not support this configuration.

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