Install 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 (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. To avoid overloading and effective
performance of the servers 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.)
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 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 maps 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 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 floods on all ports in VLAN1.
To perform load-balancing, NLB, forwards the data frame to all the servers.
NLB Multicast Mode Scenario
Consider a 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. They contain a single multicast MAC address (MAC-Cluster:
03-00-5E-11-11-11).
In Multicast NLB mode, configure a static ARP configuration command to associate the cluster IP address with a
multicast cluster MAC address.
With Multicast NLB mode, the data forwards to all the servers based on the port specified using the following Layer 2 multicast
command in CONFIGURATION MODE:
mac-address-table static <multicast_mac> multicast vlan <vlan_id> output-range <port1>,
<port2>
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