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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.
• To verify if NLB VLAN flooding is enabled, enter the show running-config command. The
command output displays the ip vlan-flooding CLI configuration, if enabled.
Configuring NLB on a Switch
You can enable NLB functionality to operate in unicast or multicast mode on a switch.
To enable NLB unicast mode:
Enter the ip vlan-flooding command to enable Layer 3 unicast data traffic routed through a
VLAN port to be flooded on all member ports of the VLAN connected to a server cluster.
CONFIGURATION mode
ip vlan-flooding
Unicast data traffic flooding is performed only on packets that use ARP entries that are resolved through
ARP packets in which the Ethernet MAC source address (SA) is different from the MAC information inside
the ARP packet.
To enable multicast NLB mode:
1. Configure a L2 multicast configuration to associate the cluster MAC address and a subset of ports
within a VLAN.
CONFIGURATION mode
mac-address-table static multicast-mac-address vlan vlan-id output-range
interface
2. Configure a static ARP entry to associate the cluster IP address with the corresponding multicast NLB
MAC address. Specify any of the interfaces entered in the L2 multicast configuration in Step 1.
CONFIGURATION mode
arp ip-address multicast-mac-address interface
Microsoft Network Load Balancing
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