Reference Guide

Microsoft Network Load Balancing
Network Load Balancing (NLB) is a clustering functionality that is implemented by Microsoft on Windows 2000 Server and
Windows Server 2003 operating systems. Microsoft NLB clustering allows multiple servers running Microsoft Windows to be
represented by one MAC and one IP address to provide transparent failover and load-balancing. The Dell Networking OS does
not recognize server clusters by default; you must configure NLB functionality on a switch to support server clusters.
Topics:
• NLB Unicast and Multicast Modes
• NLB Benefits
• NLB Restrictions
• NLB VLAN Flooding
• Configuring NLB on a Switch
NLB Unicast and Multicast Modes
On a switch, you can configure NLB functionality to operate in two modes: unicast and multicast mode.
The server-cluster IP address and the associated cluster MAC address are configured in the NLB application running on the
Windows Server.
● In unicast mode, when the server IP address is resolved to the MAC address using the ARP application, the switch
determines whether the ARP reply obtained from the server is of an NLB type. The switch then maps the IP address (cluster
IP) with the MAC address (cluster MAC address).
● In multicast mode, the cluster IP address is mapped to a cluster multicast MAC address that is configured using the static
ARP CLI configuration command. After the static NLB entry is configured, the traffic is forwarded to the subset of ports
configured for the VLAN that corresponds to the cluster virtual IP address.
NLB Unicast Mode Example
Consider a sample topology in which four servers, namely S1 through S4, are configured as a cluster or a farm. This set of
servers is connected to a Layer 3 switch, which in turn is connected to the end-clients. The servers contain a single IP address
(IP-cluster address of 172.16.2.20) and a single unicast MAC address (MAC-Cluster address of 00-bf-ac-10-00-01) for load-
balancing. Because multiple ports of a switch cannot learn a single MAC address, the servers are assigned with MAC addresses
of MAC-s1 to MAC-s4) respectively on S1 through S4 in addition to the MAC cluster address. All the servers of the cluster
belong to the VLAN named VLAN1.
In unicast NLB mode, the following sequence of events occurs:
● The switch sends an ARP request to resolve the IP address to the cluster MAC address.
● The NLB server responds with an ARP reply containing the MAC cluster address in the ARP header and a MAC address of
MAC-s1/s2/s3/s4 (for servers S1 through S4) in the Ethernet header.
● The switch associates the IP address with the MAC cluster address with the last ARP response it obtains. Assume that in
this case, the last ARP reply is obtained from MAC-s4.(assuming that the ARP response with MAC-s4 is received as the last
one). The interface associated with server, S4, is added to the ARP table.
● After the NLB ARP entry is learned on a switch when NLB enabled, all subsequent traffic is flooded on all ports in VLAN1.
With NLB, the data frame is forwarded to all servers in the cluster for the servers to perform load-balancing.
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