Administrator Guide

With Multicast NLB mode, the data forwards to all the servers based on the port specied using the following Layer 2 multicast command
in CONFIGURATION MODE:
mac-address-table static <multicast_mac> multicast vlan <vlan_id> output-range <port1>, <port2>
Limitations of the NLB Feature
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 NLB is applicable and 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 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 Networking OS does not recognize server clusters by default; you must congure 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 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, use the ip vlan-flooding command on the Dell switch when you congure 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 trac destined for the cluster is ooded out of all member ports. Because 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 you enable VLAN ooding, are deleted when you disable VLAN ooding, and RP2 triggers an ARP
resolution. Disable VLAN ooding with the no ip vlan-flooding command.
• When you add a port to the VLAN, the port automatically receives trac if you enabled VLAN ooding. 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 trac.
• There is no impact on the conguration from saving the conguration.
• If you enable VLAN ooding, it displays in the show running-config command output that displays the ip vlan-flooding
CLI conguration. This is the only output where you see the VLAN ooding status (enabled or disabled).
Conguring a Switch for NLB
To enable a switch for Unicast NLB mode, perform the following steps:
Microsoft Network Load Balancing
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