Configure NLB for Exchange 2010 - unicast or multicast

[guest]

We are in the process of configuring CAS on VSphere and want to setup NLB. The documents I have read talk about using either unicast or multicast modes. Unicast requires more configuration on the server side and multicast requires additional configuration on the network side. Is one mode better than the other?


Any assistance would be much appreciated!

[guest]

multicast is usually the preferred route due to the configuration change required to the vSphere host when using multicast mode. The change required disables ESX from updating the upstream switches when a virtual machine comes online. What that means is that the switches won't learn where the NLB virtual machines are located causing them to flood the switches, which is by design how NLB works. A side affect of this is that the switches will also not learn about changes due to vMotion or HA events which will prolong the time it takes switches to learn of the new location for a MAC address.


The multicast route requires a static entry at the network layer and does not have the side affect stated above.

[guest]

A Windows NLB array can be configured in either unicast or multicast mode. Since WNLB is not something Exchange administrators or consultants deal with on a daily basis, it can be difficult to decide which mode to choose when time comes to load balancing the RPC traffic to the RPC Client Access service on the CAS servers in the Exchange 2010 infrastructure.

So let’s take a look at each WNLB mode:

Unicast Mode
With the WNLB cluster configured in unicast mode, the MAC address of each server’s network adapter will be changed to a virtual cluster MAC address, which is the MAC address that will be used by all servers in the Windows NLB cluster. When unicast mode is enabled, clients can only connect to the servers via the VIP address on the network interface card (NIC) that has been configured with the cluster MAC address.

Multicast mode
With the Windows NLB cluster configured in multicast mode, a multicast MAC address is added to the cluster adapter of each server in the cluster. Note that I write “is added”, as each server will retain their original MAC address.

A Windows NLB cluster, no matter what mode it is configured in, works with just a single network adapter installed in each server, but it is recommended to install a second network adapter in each server, in order to achieve optimal performance, and to separate ordinary and cluster related network traffic.

So what mode should I use for my Exchange 2010 CAS array and how many network adapters should I install in each Client Access server? Well, a best practice recommendation is to install two network adapters and use unicast mode, so that the host and cluster network traffic are separated on their own respective network interface. However, if you only have the option of installing one NIC in each CAS server or if you’re forced to using multi-cast mode because of the switches used in your organization, you should pick multicast mode.

[guest]

Selecting the Unicast Method
In the unicast method:

The cluster adapters for all cluster hosts are assigned the same unicast MAC address.


The outgoing MAC address for each packet is modified, based on the cluster host’s priority setting, to prevent upstream switches from discovering that all cluster hosts have the same MAC address.

The modification of the outgoing MAC address is appropriate for switches. When a hub is used to connect the cluster hosts, disable the modification of the outgoing MAC address. On Windows Server 2003, you can disable modification of outgoing addresses by setting the value of the registry entry MaskSourceMAC, of data type REG_DWORD, to 0x0. MaskSourceMAC is located in HKLM\SYSTEM\CurrentControlSet\Services\WLBS\Parameters\Interface\Adapter-GUID (where Adapter-GUID is the long GUID assigned to the network adapter in the server).

Caution


Do not edit the registry unless you have no alternative. The registry editor bypasses standard safeguards, allowing settings that can damage your system, or even require you to reinstall Windows. If you must edit the registry, back it up first and see the Windows Server 2003 Resource Kit Registry Reference on the Microsoft Windows Server 2003 Deployment Kit companion CD or at http://www.microsoft.com/reskit.


The unicast MAC address is derived from the cluster’s IP address to ensure uniqueness outside the cluster hosts.


Communication between cluster hosts, other than Network Load Balancing–related traffic (such as heartbeat), is only available when you install an additional adapter, because the cluster hosts all have the same MAC address.


Although the unicast method works in all routing situations, it has the following disadvantages:

A second network adapter is required to provide peer-to-peer communication between cluster hosts.


If the cluster is connected to a switch, incoming packets are sent to all the ports on the switch, which can cause switch flooding.


Selecting the Multicast Method
In the multicast method:

The cluster adapter for each cluster host retains the original hardware unicast MAC address (as specified by the hardware manufacture of the network adapter).


The cluster adapters for all cluster hosts are assigned a multicast MAC address.


The multicast MAC is derived from the cluster’s IP address.


Communication between cluster hosts is not affected, because each cluster host retains a unique MAC address.


By using the multicast method with Internet Group Membership Protocol (IGMP), you can limit switch flooding, if the switch supports IGMP snooping. IGMP snooping allows the switch to examine the contents of multicast packets and associate a port with a multicast address. Without IGMP snooping, switches might require additional configuration to tell the switch which ports to use for the multicast traffic. Otherwise, switch flooding occurs, as with the unicast method.

The multicast method has the following disadvantages:

Upstream routers might require a static Address Resolution Protocol (ARP) entry. This is because routers might not accept an ARP response that resolves unicast IP addresses to multicast MAC addresses.


Without IGMP, switches might require additional configuration to tell the switch which ports to use for the multicast traffic.


Upstream routers might not support mapping a unicast IP address (the cluster IP address) with a multicast MAC address. In these situations, you must upgrade or replace the router. Otherwise, the multicast method is unusable.

[guest]

What is best to use for NLB, multicast or unicast? We have 2 x Windows 2008
servers each have 4 port nics.

[guest]

Hears a mess of stuff on that. These are in the context of ISA2006 Arrays
with NLB,...but NLB is still NLB no matter how you cut it. One of these
talks a lot about Multicast -vs- Unicast, I just do not remember which one
(maybe the second one).

Me, Myself and ISA Blog (MSFirewall.org.uk): ISA Server 2006/2004
Configuration Storage Server Frequently Asked Questions
http://blog.msfirewall.org.uk/2009/04/i ... ation.html

Me, Myself and ISA Blog (MSFirewall.org.uk): Resource Guide for Using
Microsoft NLB with ISA Server 2006 Enterprise Edition
http://blog.msfirewall.org.uk/2008/10/r ... t-nlb.html


--
Phillip Windell

[guest]

Again, I do not know the exactly reason in your setup for using multicast
here, but I personally would not use it due to generated network traffic.
at msexchange.org you can find an article which discusses this as well :
http://www.msexchange.org/articles_tuto ... part1.html


Normally you should have 2 adapters installed and have seperated networks
for management and public traffic. the adapters from both server which are
connected to the public network should then be configured for NLB, again
ideally in Unicast mode :-)

Check out the following link to get a more details around windows 2008 NLB
features :

NLB - Deployment Guide
http://technet.microsoft.com/en-us/libr ... 4833(WS.10).aspx

Network Load Balancing Step-by-Step Guide: Configuring Network Load
Balancing with Terminal Services
http://technet.microsoft.com/en-us/libr ... 1300(WS.10).aspx

NLB - Overview
http://technet.microsoft.com/en-us/libr ... 25691.aspx