Port rules, shown in Figure 16-5, control, on a port-by-port basis, how network traffic is treated by an NLB cluster. By default, the cluster balances all traffic received on the cluster IP address across all nodes. You can modify this so that only specific traffic, designated by port, received on the cluster IP address is balanced. The cluster drops any traffic that does not match a port rule. You can also configure the cluster to forward traffic to a specific node rather than to all nodes, enabling the cluster to balance some traffic but not all traffic. You accomplish this by configuring the port rule's filtering mode. The options are multiple host or single host.
For example, you might configure four hosts to be part of an NLB cluster for a Web site. One of these hosts might also function as an SMTP server. Although you want the cluster to balance incoming Web traffic, you want only one host to handle SMTP traffic. To support this configuration, you create two port rules. The first would direct Transmission Control Protocol (TCP) traffic on ports 80 and 443 to all hosts in the cluster. The second port rule would direct
SMTP traffic on port 25 to a single host, the one that has the SMTP service installed. The first port rule would use the multiple host filtering mode, and the second port rule would use the single host filtering mode.
When you configure a rule to use the multiple host filtering mode, you can also configure the rule's affinity property. The affinity property determines where the cluster will send subsequent client traffic after the initial client request. If you set the affinity property to Single, the cluster will tie all client traffic during a session to a single node. This is useful for applications such as e-commerce Web sites on which multiple client-server transactions occur over a session. The default port rule, shown in Figure 16-6, uses the Single affinity setting. When you set a rule's affinity property to None, the cluster will not bind a client session to any particular node. When you set a rule's affinity property to Network, a client session will be directed to cluster nodes located on a specific TCP/IP subnet. It is not necessary to configure the affinity for a single host rule because that rule already ties traffic to a single node in the cluster.
You can edit the load placed on each node by editing port rules on each node of the cluster. Editing the load changes the load from balanced between all nodes to preferring one node or several nodes over other nodes. Do this when the hardware or one or more nodes have greater capacity than other nodes. You configure port rules in the practice at the end of this lesson.
When you need to perform maintenance on a node in an NLB cluster, you can use the Drain function to stop new connections to the node without disrupting existing connections. When all existing connections have finished, you can then take the cluster offline for maintenance. You can drain a node by right-clicking it from within Network Load Balancing Manager, clicking Control Ports, and then clicking Drain.
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