Comparing Unicast and Multicast NLB Cluster Operation Modes

The cluster operation mode is used to ensure that all cluster hosts receive all incoming client requests sent to the virtual IP address assigned to the cluster

Method

Description

Unicast

• All cluster hosts share an identical unicast media access control (MAC) address

• The outgoing MAC address is modified, based on the cluster host's priority setting, to prevent switches from discovering that all cluster hosts have the same MAC address

Multicast

• Each cluster host retains the original adapter MAC address

• The adapter is assigned a multicast MAC address, which is shared by all cluster hosts

Key Points

We recommend that you select the unicast method for distributing client requests, unless only one network adapter is installed in each cluster host.

Considerations for using unicast mode include:

• In order to provide peer-to-peer communication between cluster hosts, a second network adapter is required.

• If you connect the cluster to a switch, incoming packets are sent to all the ports on the switch, which may cause switch flooding. Outgoing packets use a modified version of the cluster MAC address that is unique to each node. This prevents switches from associating the cluster MAC address to a specific switch port.

Considerations for using the multicast mode include:

• Upstream routers might require a static Address Resolution Protocol (ARP) entry. This is because routers might not accept an ARP response that resolves unicast IP addresses to multicast MAC addresses.

• The Internet Group Management Protocol (IGMP) multicast check box enables IGMP support for limiting switch flooding, by limiting traffic to Network Load Balancing ports only. Enabling IGMP support ensures that traffic intended for an NLB cluster passes through only those ports serving the cluster hosts, and not all switch ports. Without IGMP, switches might require additional configuration to tell the switch which ports to use for the multicast traffic.

• Upstream routers might not support mapping a unicast IP address (the cluster IP address) with a multicast MAC address. In these situations, you must upgrade or replace the router, or use the unicast mode.

Options for Configuring Host Parameters

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