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Limitations of the NLB Feature
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 NLB is applicable
and 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.
Microsoft Clustering
To provide transparent failover or balancing, Microsoft clustering allows multiple servers using Microsoft Windows to be
represented by one MAC address and IP address. The Dell EMC Networking OS does not recognize server clusters by default;
you must configure it 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 EMC 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.
To preserve failover and balancing, the switch forwards the traffic destined for the server cluster to all member ports in the
VLAN connected to the cluster. To ensure that this happens, use the ip vlan-flooding command on the Dell EMC switch
when you configure the Microsoft cluster. The server MAC address is given in the Ethernet frame header of the ARP reply,
while the virtual MAC address representing the cluster is given in the payload. Then, all the traffic destined for the cluster is
flooded out of all member ports. Because all the servers in the cluster receive traffic, failover and balancing are preserved.
Enable and Disable VLAN Flooding
● The older ARP entries are overwritten whenever newer NLB entries are learned.
● All ARP entries, learned after you enable VLAN flooding, are deleted when you disable VLAN flooding, and RP2 triggers an
ARP resolution. Disable VLAN flooding with the no ip vlan-flooding command.
● When you add a port to the VLAN, the port automatically receives traffic if you enabled VLAN flooding. Old ARP entries are
not deleted or updated.
● When you delete a member port, its ARP entries are also deleted from the content addressable memory (CAM).
● Port channels in the VLAN also receive traffic.
● There is no impact on the configuration from saving the configuration.
● If you enable VLAN flooding, it displays in the show running-config command output that displays the ip vlan-
flooding CLI configuration. This is the only output where you see the VLAN flooding status (enabled or disabled).
Topics:
• Configuring a Switch for NLB
Configuring a Switch for NLB
To enable a switch for Unicast NLB mode, perform the following steps:
● Enter the ip vlan-flooding command to specify that all Layer 3 unicast routed data traffic going through a VLAN
member port floods across all the member ports of that VLAN.
CONFIGURATION mode
ip vlan-flooding
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Microsoft Network Load Balancing