Administrator Guide

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.
Limitations With Enabling NLB on
Switches
The following limitations apply to switches on which you configure NLB:
• The NLB unicast mode uses switch flooding to transmit all packets to all the servers that are part of the
VLAN. When a large volume of traffic is processed, the clustering performance might be impacted in a
small way. This limitation is applicable to switches that perform unicast flooding in the software.
• The ip vlan-flooding command applies globally across the system and for all VLANs. In cases where
the NLB is applicable and the ARP replies contain a discrepancy in the Ethernet SHA and ARP header
SHA frames, a flooding of packets over the relevant VLAN occurs.
• The maximum number of concurrent clusters that is supported is eight.
Benefits and Working of Microsoft
Clustering
Microsoft clustering allows multiple servers using Microsoft Windows to be represented by one MAC address
and IP address in order to provide transparent failover or balancing. Dell Networking OS does not recognize
server clusters by default; it must be configured to do so. When an ARP request is sent to a server cluster,
either the active server or all the servers send a reply, depending on the cluster configuration. If the active
server sends a reply, the Dell switch learns the active server’s MAC address. If all servers reply, the switch
registers only the last received ARP reply, and the switch learns one server’s actual MAC address; the virtual
MAC address is never learned. Because the virtual MAC address is never learned, traffic is forwarded to only
one server rather than the entire cluster, and failover and balancing are not preserved.
Microsoft Network Load Balancing 571