Users 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. NLB uses a distributed methodology or pattern to equally split and balance the network trac 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 for the transmission of network trac. At the same time, it also uses a
single virtual IP address for all clients as the destination IP address, which enables servers to join the same multicast group in a way 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. The NLB functionality enables ooding of trac over the VLAN ports (for unicast mode) or a subset of ports in a
VLAN (for multicast mode) to avoid overloading and eective performance of the servers for optimal processing of data packets. The
maximum NLB entry limit from 8 to 11 is increased and support for more CAM-ACL to increase.
NLB functions in two modes, namely unicast mode and multicast mode. The cluster IP address and the associated cluster MAC address are
congured in the NLB application running on the Windows Server. In the unicast mode, when the server IP address is attempted to be
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 congured using a static ARP CLI conguration command. After the NLB
entry is learned, the trac is forwarded to all the servers in the VLAN corresponding to the cluster virtual IP address.
NLB Unicast Mode Scenario
Consider a sample topology in which four servers, namely S1 through S4, are congured 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 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 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 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 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.
• With NLB feature enabled, after learning the NLB ARP entry, all the subsequent trac is ooded on all ports in VLAN1.
With NLB, the data frame is forwarded to all the servers for them to perform load-balancing.
NLB Multicast Mode Scenario
Consider a sample topology in which four servers, namely S1 through S4, are congured 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. They contain a single multicast MAC address (MAC-Cluster:
03-00-5E-11-11-11).
In the multicast NLB mode, a static ARP conguration command is congured to associate the cluster IP address with a multicast cluster
MAC address.
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