This write-up describes how the various Network pack Balancing (NLB) operation modes impact network infrastructure, and the finest ways in which the network can support each mode.

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Applies to: windows Server 2016, home windows Server 2012 R2Original KB number: 4494444


Based on our experience, the most usual issue that users encounter once they use NLB is the they space not informed enough around the technology. Therefore, the deployments and implementations usually lack some obligated settings, or don\"t take into consideration the most vital factor in every network: Bandwidth consumption.

NLB can operate in any type of of 3 modes: unicast, multicast, and multicast that uses Internet group Membership Protocol (IGMP multicast). Every of these modes has different requirements and also places different demands on the network infrastructure.


All that the nodes in a cluster must use the same procedure mode.

The adhering to table summarizes the requirements, advantages, and disadvantages of every mode.

NLB procedure modeSpecial requirementsAdvantagesDisadvantages
UnicastNLB need to be may be to adjust the MAC adapter address.Easy come configure ideal for basic environments May flood various other systems v network traffic, bring about performance issues (you may need to use additional hardware to resolve those issues) Not ideal for more complex environments
MulticastThe network facilities must usage a revolution ARP entry and a static MAC deal with table entry.More effective use of bandwidth and also lower risk of performance impacts than unicast mode Each adapter supplies its integrated MAC resolve More complex to configure than unicast
Multicast with IGMPThe network switches have to be capable of IGMP snooping.Same benefits as multicast additional advantage of automatic configuration. Requires the the network hardware have certain capabilities that the other settings do no need

You deserve to configure an NLB cluster in among three operation modes: Unicast, multicast, or IMGP multicast. All three settings work very well if your framework is properly configured. However, serious problems can construct if friend haven\"t prepared your network facilities to support the setting that you room using. Each mode has various implications for the network infrastructure.


Unicast is the easiest operation mode to configure. In theory, girlfriend don\"t have to do anything else in her network infrastructure. In actuality, you may have to modify the facilities to manage network traffic.

In unicast mode, NLB supplies the NLB MAC address to change the initial hardware MAC deal with of every adapter in every node the the cluster. Due to the fact that multiple adapters now have actually the same address, any kind of physical switches in the network have the right to no longer correctly maintain their MAC resolve tables. Because they cannot recognize which web traffic goes to which move port, the switches start sending out all website traffic to every ports come make certain that the website traffic reaches its destination. This is known as a unicast flood scenario.

A unicast flood have the right to seriously affect the network performance. In addition to the continuous network traffic, every NLB node sends out out a heartbeat packets (each heartbeat packet contains around 1500 bytes of data). By default, a node sends a heartbeat packet each second and waits for 5 of those packets to be obtained until that considers the node as converged. In a unicast flood situation, any switches rebroadcast this heartbeat traffic to all switch ports just like the continuous network traffic. Because that example, if your network has actually a 24-port or 48-port switch, and also only 2 of those ports connect to NLB nodes, the move may end up broadcasting far-ranging network traffic to 22 (or 46) servers the don\"t require it.

To stop a unicast flood, you have the complying with options:

Option 1: Insert a hub between the network switch and the NLB nodes. The hub uses the NLB unicast MAC resolve and connects come a single switch port, therefore the switch deserve to correctly control its MAC deal with table. The hub forwards traffic to the NLB nodes, and also servers that affix to the various other switch ports don\"t obtain the extra NLB traffic.

Option 2: develop a different VLAN for the NLB servers. Make sure that other subnets can reach the VLAN. This configuration isolates NLB web traffic to the move ports that space assigned to the VLAN.

Configure computer systems that have dual NICs in unicast mode

In part cases, you desire to have two network interface cards (NICs) on her computer. If you are running home windows Server 2008 or later, girlfriend must enable IP forwarding top top the NICs in order to ensure that traffic gets routed properly. IP forwarding is allowed by default in previously versions the Windows.

Before you permit IP forwarding, you have to obtain the index of the cluster NIC. On the computer that you want to configure, open a Command Prompt window and operation the adhering to command:

netsh interface ipv4 show intThe calculation of this command perform the interfaces on the computer, as follows.


In the Command prompt window, operation the adhering to command:

netsh interface ipv4 set interface forwarding=enabledIn this command, represents the index of the swarm interface.

To verify that the setting has changed, run the complying with command:

netsh interface ipv4 show interface l=verboseIn this command, represents the index of the swarm interface.

The output mirrors that forwarding is currently enabled.


Configure virtual settings in unicast mode

By default, the digital switches in virtual settings usually prevent unicast flooding. For much more configuration information, see the adhering to resources:

If you room using another virtual setting (such as XenServer or VirtualBox) and also you are experiencing similar issues, contact the manufacturer because that guidance.


Multicast setting differs indigenous unicast mode. Rather of changing the MAC addresses top top the network adapters, NLB converts the NLB digital IP (VIP) address to one NLB multicast MAC address. This MAC has the format of 03-BF-XX-XX-XX-XX. NLB additionally makes certain that the cluster\"s main IP deal with resolves to this multicast resolve as part of the address Resolution Protocol (ARP). Although the separation, personal, instance network adapters maintain their initial MAC addresses, the NLB traffic is handle to the NLB multicast MAC address.

To support this configuration, you should configure the network framework to use static ARP entries and also MAC deal with table entries. Network switches cannot learn the NLB multicast MAC attend to in the course of your usual operations. If girlfriend skip the hands-on configuration step, the network switches might flood NLB web traffic to every ports or autumn packets. The network might seem to role correctly in ~ first, yet problems rise over time.

The posts that are detailed in the adhering to table explain plainly what you have to do to appropriately configure your network infrastructure, based on your network framework vendor. Psychic that we don\"t keep these articles. Therefore, us cannot guarantee the they are accurate or available. If you have any questions about these articles, please call the proper vendor.

VMwareSample configuration - Network load Balancing (NLB) Multicast setting over routed subnet - Cisco Switch static ARP configuration (1006525)
HPEHP move 5500/5500G - how to implement Network load Balancing using multicasts top top the switch 5500 and 5500G
HuaweiExample for Connecting a machine to an NLB swarm (Using Multi-Interface ARP)
D-LinkD-Link great 3 switch NLB in Multicast setting Configuration Example
AvayaTechnical Configuration guide for Network load Balancing (download)
H3C05-Layer 3 - IP solutions Configuration Guide

Configure virtual settings in multicast mode

In a virtual environment, the network switches attach to the hypervisor organize servers. In a high-availability digital environment, a team of hypervisor hosts supports a team of digital machines. An individual virtual an equipment may reside on any of the hypervisor hosts, and also it might migrate to a various hypervisor host under certain circumstances. The network traffic must be able to reach the correct virtual machine regardless of i beg your pardon hypervisor organize that virtual an equipment runs on.

To usage multicast mode in such an environment, you need to configure the MAC attend to tables of the network switches so the each port that connects to a hypervisor hold uses a static entry to map come the NLB multicast MAC address. For example, think about an environment that has eight hypervisor hosts. Each hypervisor host has actually two network adapters, and all of the adapters affix to a switch. The MAC address table for the switch needs static entries that map each harbor to the NLB Multicast MAC address.

IGMP multicast

To use IGMP multicast, the network switches have to be qualified of IGMP snooping.

This setting is usually the exact same as multicast mode, other than that the switches can instantly build your MAC attend to tables in this mode.

When you enable IGMP multicast, the NLB nodes send IGMP sign up with messages to the 239.255.x.y multicast address (in this address, x.y to represent the last 2 octets the the NLB VIP). Because that example, if the NLB VIP is, the multicast address for IGMP sign up with messages is These messages indicate the team membership of the NLB nodes. The switches usage this info to configure the MAC address table.

Some the the write-ups that are detailed in the Multicast section encompass the correct construction parameters because that their devices for multicast v IGMP. Come verify the your devices can assistance this mode, call your hardware vendor.

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Configure the NLB procedure mode

From an NLB standpoint, the configuration is straightforward: download the role, open up the console, develop a load-balancing cluster, select the nodes, set the NLB mode, and also then set the ports and affinity.