Concept Guide

With multicast NLB mode, the data is forwarded to all the servers based on the port specied 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 congure NLB:
• The NLB unicast mode uses switch ooding to transmit all packets to all the servers that are part of the VLAN. When a large volume of
trac is processed, the clustering performance might be impacted in a small way. This limitation is applicable to switches that perform
unicast ooding 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 ooding of packets over the relevant VLAN
occurs.
• The maximum number of concurrent clusters that is supported is 128.
Benets 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 congured 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
conguration. 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, trac 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 trac destined for the server cluster to all member ports in the VLAN
connected to the cluster. To ensure that this happens, you must congure the ip vlan-flooding command on the Dell switch at the
time that the Microsoft cluster is congured. 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 trac destined for the cluster is ooded out of all
member ports. Since all the servers in the cluster receive trac, 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 the feature is enabled, are deleted when the feature is disabled, and RP2 triggers an ARP resolution. The
feature is disabled with the no ip vlan-flooding command.
• When a port is added to the VLAN, the port automatically receives trac if the feature is enabled. Old ARP entries are not deleted or
updated.
• When a member port is deleted, its ARP entries are also deleted from the CAM.
• Port channels in the VLAN also receive trac.
• There is no impact on the conguration from saving the conguration.
• The feature, if enabled, is displayed in the show running-config command output that displays the ip vlan-flooding CLI
conguration. Apart from it, there is no indication of the enabling of this capability.
Conguring a Switch for NLB
To enable a switch for unicast NLB mode of functioning, perform the following steps:
Enter the ip vlan-flooding command to specify that all Layer 3 unicast routed data trac, going through a VLAN member port,
needs to be ooded across all the member ports of that VLAN. There might be some ARP table entries that are resolved through ARP
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Microsoft Network Load Balancing