Reference Guide

NLB Multicast Mode Example
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.
In multicast NLB mode, data is forwarded to all servers in the cluster based on the port specified using the Layer 2 multicast
command: mac-address-table static <multicast_mac> multicast vlan <vlan_id> output-range
<port1>, <port2>, ... in CONFIGURATION mode.
NLB Benefits
You must configure a switch to recognize Microsoft NLB clustering so that multiple servers using Microsoft Windows can be
represented by one MAC address and IP address to support transparent server failover and load-balancing.
When NLB functionality is not enabled and a switch sends an ARP request 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 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; server failover and balancing are not supported.
To preserve server failover and balancing, the switch forwards traffic destined to the server cluster on all member ports in the
VLAN connected to the cluster. To configure this switch capability, enter the ip vlan-flooding command when you
configure the Microsoft server cluster.
The server MAC address is given in the Ethernet frame header of the ARP reply, while the virtual MAC address of the cluster is
given in the payload. As a result, all traffic destined for the server cluster is flooded from the switch on all VLAN member ports.
Since all servers in the cluster receive traffic, failover and load-balancing are preserved.
NLB Restrictions
The following limitations apply to switches which support Microsoft network load balancing.
● NLB unicast mode uses switch flooding to transmit packets to all servers that are part of the VLAN connected to the
cluster. 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 all VLANs on the switch. In cases where NLB VLAN flooding is
enabled and ARP replies contain a discrepancy in the Ethernet SA and ARP header SA frames, packet flooding over the
relevant VLAN is performed.
● The maximum number of server clusters supported at a time is eight.
NLB VLAN Flooding
To preserve Microsoft server failover and load-balancing, configure a switch to forward the traffic destined for a server cluster
on all member ports of the VLAN connected to the cluster (ip vlan-floodingcommand). Configure the switch for NLB
VLAN flooding when you configure the server cluster.
After you configure a switch to perform NLB VLAN flooding:
● Older ARP entries are overwritten when newer NLB entries are learned. All learned ARP entries are deleted when you disable
NLB VLAN flooding (no ip vlan-flooding command).
● When you add a port to the NLB VLAN, the port automatically receives traffic if the feature is enabled. Old ARP entries are
not deleted or updated. Port channels in the NLB VLAN also receive traffic. When you delete a VLAN member port, its ARP
entries are also deleted from CAM.
● There is no impact on the running configuration if you save the switch configuration with NLB VLAN flooding enabled.
Microsoft Network Load Balancing
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