Administrator Guide

Table Of Contents
Microsoft Network Load Balancing
This functionality is supported on Dell Networking OS.
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 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 for the transmission
of network traffic. 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 flooding of traffic over the 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 for optimal processing of data packets.
NLB functions in two modes, namely unicast mode and multicast mode. The cluster IP address and the associated cluster MAC
address are configured 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 configured using
a static ARP CLI configuration command. After the NLB entry is learned, the traffic 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 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
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 traffic is flooded 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 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. They contain a single multicast MAC
address (MAC-Cluster: 03-00-5E-11-11-11).
In the multicast NLB mode, a static ARP configuration command is configured to associate the cluster IP address with a
multicast cluster MAC address.
With multicast NLB mode, the data is forwarded to all the servers based on the port specified using the Layer 2 multicast
command, which is the mac-address-table static <multicast_mac> multicast vlan <vlan_id> output-
range <port1>, <port2> command in CONFIGURATION mode.
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