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NLB Multicast Mode Example
Consider a sample topology in which four servers, namely S1 through S4, are configured as a cluster or a farm. This set of
servers is connected to a Layer 3 switch, which in turn is connected to the end-clients. They contain a single multicast MAC
address (MAC-Cluster: 03-00-5E-11-11-11).
In the multicast NLB mode, a static ARP configuration command is configured to associate the cluster IP address with a
multicast cluster MAC address.
In multicast NLB mode, data is forwarded to all servers in the cluster based on the port specified using the Layer
2 multicast command: mac-address-table static <multicast_mac> multicast vlan <vlan_id> output-
range <port1>, <port2>, ... in CONFIGURATION mode.
NLB Benefits
You must configure a switch to recognize Microsoft NLB clustering so that multiple servers using Microsoft Windows can be
represented by one MAC address and IP address to support transparent server failover and load-balancing.
When NLB functionality is not enabled and a switch sends an ARP request 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 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; server failover and balancing are not supported.
To preserve server failover and balancing, the switch forwards traffic destined to the server cluster on all member ports in
the VLAN connected to the cluster. To configure this switch capability, enter the ip vlan-flooding command when you
configure the Microsoft server cluster.
The server MAC address is given in the Ethernet frame header of the ARP reply, while the virtual MAC address of the cluster is
given in the payload. As a result, all traffic destined for the server cluster is flooded from the switch on all VLAN member ports.
Since all servers in the cluster receive traffic, failover and load-balancing are preserved.
NLB Restrictions
The following limitations apply to switches which support Microsoft network load balancing.
● NLB unicast mode uses switch flooding to transmit packets to all servers that are part of the VLAN connected to the
cluster. 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 all VLANs on the switch. In cases where NLB VLAN flooding
is enabled and ARP replies contain a discrepancy in the Ethernet SA and ARP header SA frames, packet flooding over the
relevant VLAN is performed.
● The maximum number of server clusters supported at a time is eight.
● When you reconfigure CAM allocation, use the nlbclusteracl number command to change the number of NLB ARP
entries. The range is from 0 to 2. The default value is 0. At the default value of 0, eight NLB ARP entries are available for
use. This platform supports upto 512 CAM entries. Select 1 to configure 256 entries. Select 2 to configure 1024 entries. Even
though you can perform CAM carving to allocate the maximum number of NLB entries, Dell Networking recommends you to
use a maximum of 64 NLB ARP entries.
● If the following error appears, then the maximum allocated CAM space is utilized: ACL_AGENT-4-
ACL_AGENT_CAMSIZE_WARNING: L3 CAM (NLB) on portpipe 0 stack-unit 0 is Full. You must re-
configure CAM by selecting option 2 that configures 1024 CAM entries.
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Microsoft Network Load Balancing