Service Manual

With NLB, the data frame forwards to all the servers for them to perform load-balancing.
NLB Multicast Mode Scenario
Consider a sample 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
Microsoft clustering allows multiple servers using Microsoft Windows to be represented by one MAC
address and IP address to provide transparent failover or balancing. 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, you must configure 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 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.
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Microsoft Network Load Balancing