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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 (OSs). NLB uses a distributed methodology or
pattern to equally split and balance the network traffic load across a set of servers that are part of the
cluster or group. NLB combines the servers into a single multicast group and attempts to use the
standard multicast IP or unicast IP addresses, and MAC addresses to transmit of network traffic. At the
same time, NLB also uses a single virtual IP address for all clients as the destination IP address, which
enables servers to join the same multicast group that is transparent to the clients (the clients do not
notice the addition of new servers to the group). The clients use a cluster IP address to connect to the
server. For optimal processing of data packets, NLB enables flooding of traffic over the virtual local area
network (VLAN) ports (for Unicast mode) or a subset of ports in a VLAN (for Multicast mode) to avoid
overloading and effective performance of the servers.
NLB functions in two modes, Unicast mode and Multicast mode. Configure the cluster IP address and the
associated cluster MAC address in the NLB application running on the Windows Server.
• In Unicast mode, when the server IP address attempts to be resolved to the MAC address using the
address resolution protocol (ARP), 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 you
configured using a static ARP command. After the NLB entry is learned, the traffic forwards to all
the servers in the VLAN corresponding to the cluster virtual IP address.
NLB Unicast Mode Scenario
Consider a sample topology in which you configure four servers, S1 through S4, as a cluster or a farm.
This set of servers connects to a Layer 3 switch, which connects 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 on a switch cannot learn a
single MAC address, the servers are assigned MAC addresseses 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 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 ARP servers send an ARP response with 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 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.
• With NLB enabled, after learning the NLB ARP entry, all the subsequent traffic is flooded on all ports in
VLAN1.
Microsoft Network Load Balancing
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