Nlb Multicast Mode Scenario; Limitations Of The Nlb Feature; Microsoft Clustering - Dell S4048T Configuration Manual

NLB Multicast Mode Scenario

Consider a topology in which you configure four servers, S1 through S4, as a cluster or a farm. This set of
servers connects to a Layer 3 switch, which connects to the end-clients. They contain a single multicast MAC
address (MAC-Cluster: 03-00-5E-11-11-11).
In Multicast NLB mode, configure a static ARP configuration command to associate the cluster IP
address with a multicast cluster MAC address.
With Multicast NLB mode, the data forwards to all the servers based on the port specified 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 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 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 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 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
Microsoft Network Load Balancing 638