Using Network Load Balancing in unicast mode and allowing the servers to communicate

Posted on 2009-02-19
Last Modified: 2012-05-06

Taking this question a step further,

IS there a way to make two servers in a cluster communicate with each other while using Unicast with one NIC?  Two NICS?

We have two front end Windows 2003 Exchange 2003 servers in an NLB using unicast.  There are two NICS present. NLB is bound to 1 nic on each.

Our problem is that since these are both front ends, i would think Exchange would like to pass information between these two from time to time, but instead has to route a different way.  This is noticable if you make one server a bridgehead for a connector, but not the other.  A delay will occur (knowing full well that these servers need to be configured the same since they are in an NLB cluster but that is just an example).

Do I need to change to multicast after confirming my switch will allow that?

Question by:prnamessaging
    LVL 82

    Expert Comment

    In Unicast mode, these servers won't be able to communicate with each other, because they're sharing the same MAC address on the NLB NIC.
    You'll either need to add a second NIC (which can cause other problems related to DNS registrations and multihoming; these aren't DCS?), or change to Multicast (which, on the other hand, may cause problems accessing the NLB address from outside the servers' subnet, because most routers don't publish Multicast addresses).
    LVL 2

    Author Comment

    oBdA (great name by the way),

    Not DCs. They do already have 2 active NICs each (NLB cluster only on one NIC each).

    As a test, I quickly switched the NLB cluster to multicast and after the cluster settled, it went totally offline. Was unable to ping any host or the cluster IP.

    Talking to my Networking co-worker, he stated that the ARP table on the switches problaby had the old entries and wouldn't refresh for maybe 4 hours.  We have a complete power down coming tomorrow which will take all systems down.  I'm thinking I can switch to multicast before shutdown, and when everything comes up give it a try and if all fails, go back to unicast for now.

    Regarding accessing the NLB address from outside the servers' subnet, that may be a problem.  We use Vlans to segment traffic so our subnets are few but large... however some remote sites have different subnets.  We will have to test this...

    LVL 82

    Expert Comment

    If there are two NICs in there, and the non-NLB NIC is active, then the servers should be able to communicate over this one (if they can reach each other through this connection). If you haven't done so already, you should unbind NetBIOS over TCP/IP from one of the NICs (ideally on the NLB NIC).
    Here is some further

    Network Load Balancing: Configuration Best Practices for Windows 2000 and Windows Server 2003

    Network Load Balancing Frequently Asked Questions

    How Network Load Balancing works

    Using Clustering for a Highly Available Web Site: An Example

    Network Load Balancing clusters

    Network Load Balancing Best practices
    LVL 2

    Author Comment

    NIC1 public subnet has Netbios over TCP off
    NIC2 private subnet has Netbios over TCP on

    I should explain the 2 NIC setup because that may cause your Netbios suggestion to not work.
    Cluster bound NIC1 IP address ,
    NIC2 IP address , - TSM backup system uses this NIC. This subnet cannot communicate with the NIC1 subnet. However, can ping the 172 IP address of Server2.
    Cluster bound NIC1 IP address,
    NIC2 IP address, - TSM backup system uses this NIC.  This subnet cannot communicate with the NIC2 subnet however can ping the 172 IP address of Server1.

    Cluster IP,

    The NIC2 subnet is only used locally, the TSM server is on the same range, no gateway or routing needed.

    I have reviewed those links above and I understand what I'm doing and why they cannot communicate with each other. However, my management would like to see these two servers be able to read each others mail queue information in ESM for example.  
    They understand why this happens (I showed them the assigned MAC address to all the cluster IPs) but if there is a way to let them operate as normal AND have them in a NLB cluster I need to find out.

    LVL 2

    Author Comment

    I should add, that I have the ability to activate another NIC on each machine and put that on the same subnet as NIC1. They exist but are disabled with no cable.  (each server has 4 nics). Solution?

    LVL 82

    Accepted Solution

    Adding another NIC in the 10 subnet would enable the inter-host communication, yes. In this case, disable DNS registration for all NICs.
    Instead, manually create DNS entries with the host names of the servers pointing to the "new" IP addresses, and (if required) dedicated names for the NLB NICs (and the clustered IP).
    Example:        DNS: exch1-tsm   \       DNS: exch1       | Exchange host 1       DNS: exch1-nlb   /
      +--  DNS: exch
      |       DNS: exch2-nlb   \       DNS: exch2       | Echange host 2        DNS: exch2-tsm   /

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    LVL 2

    Author Closing Comment

    Information is clear and understandable.  Thank you.

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