Administrator Guide

Limitations With Enabling NLB on Switches
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 the NLB is
applicable and the 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.
Benefits 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 configured 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 at the time that the Microsoft cluster is configured. 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. Since all the servers in the cluster receive traffic, 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 traffic 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 traffic.
● There is no impact on the configuration from saving the configuration.
● The feature, if enabled, is displayed in the show running-config command output that displays the ip vlan-
flooding CLI configuration. Apart from it, there is no indication of the enabling of this capability.
Topics:
• Configuring a Switch for NLB
Configuring a Switch for NLB
This functionality is supported on the Z9000 platform.
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 traffic, going through a VLAN member
port, needs to be flooded across all the member ports of that VLAN. There might be some ARP table entries that are resolved
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Microsoft Network Load Balancing