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  • Load Balancing Exchange SMTP Relay and IIS SMTP Relay


    danschlimmernd.png by Dan Schlimme

    I recently had to load balance our Exchange SMTP Relay and IIS SMTP Relay.  Both of these require the use of DSR which I was unfamiliar with and wanted to share my experience.

    Important Pieces of Information

    • Netscaler Firmware
      • NS 10.5 61.11.nc
    • Exchange Environment
      • Exchange 2010
    • Server OS
      • Server 2012 R2
    • Your vip needs to be on the same VLAN as your load balanced servers
    • You will need a SNIP for the VLAN of your load balanced servers
    • You will need to add the VLAN of your load balanced servers to your network port
    • MAC based forwarding needs to be enabled

    Netscaler and Server Networking configuration are the same for Exchange SMTP Relay and IIS SMTP Relay

    Netscaler Configuration

    • Add the VLAN you will be working with to your Netscaler
      • Navigate to System\Network\VLANs
        • Select Add
          • Enter the VLAN ID
          • Bind to the Interface you need
            • I tagged the traffic, but this may not be needed in your environment
    • Add SNIP to the Netscaler on the VLAN you just added
      • Navigate to System\Network\IPs
        • Select Add
          • Enter the IP
          • Enter the Netmask
          • Select IP Type as Subnet IP
          • OPTIONAL STEP - Uncheck “Enable Management Access control to support the below listed applications”
    • Add the servers you want to load balance
    • Create the SMTP Monitor
      • Standard Parameters
        • Give it a name
        • Set the port number to 25
    • Special Parameters
      • Select Script of nssmtp.pl
      • Set Dispatcher IP to
      • Set Dispatcher port to 3013
    • Create the Service Group
      • Basic Settings Configuration
        • Give it a name
        • Set Protocol to ANY
    • Settings Configuration
      • Select Use Proxy Port
      • Select Down State Flush
      • Select Use Client IP
    • Monitor Configuration
      • Add the SMTP Monitor you created earlier
      • You also need to add a second monitor of TCP/Ping/ARP
        • I used ARP
          • This has something to do with learning the MAC address. I am not entirely certain.
    • Service Group Members Configuration
      • Add the Servers you added earlier
    • Create the Virtual Server
      • Basic Settings Configuration
        • Give it a name
        • Set Protocol to ANY
        • Assign IP
        • Set Redirection Mode to MAC Based
    • Service and Service Groups Configuration
      • Bind the Service Group you created earlier
    • Method Configuration
      • Set Load Balancing Method to Round Robin
    • Persistence Configuration
      • Set Persistence to SourceIP
    • Traffic Settings Configuration
      • Select Sessionless Load Balancing
    • Your Virtual Server is now configured

    Server Configuration

    • Add a loopback network adapter
    • Rename Ethernet adapter to Ethernet
    • Rename the Loopback adapter to Loopback
    • Open properties of the Loopback adapter
      • Uncheck everything except Internet Protocol Version 4 (TCP/IPv4)
    • Select Internet Protocol Version 4 (TCP/IPv4) and select Properties
      • General Settings Configuration
        • Add the IP you used for your SMTP Virtual Server
        • Set the Subnet Mask to
        • Do not set DNS
    • Advanced Configuration Settings
      • IP Settings
        • Uncheck Automatic metric
        • Set Interface Metric to 254
          • This prevents the ip from arping
    • DNS
      • Uncheck Register this connection’s address in DNS
    • WINS
      • Select Disable NetBIOS over TCP/IP
    • From command line run the following commands with elevated privileges
      • netsh int ipv4 set int "Loopback" weakhostreceive=enabled weakhostsend=enabled
      • netsh int ipv4 set int "Ethernet" weakhostreceive=enabled
      • arp -d *
    • Your Server is now configured for DSR

    IIS SMTP Relay Configuration

    • Open IIS on your Relay server
    • Go to properties of your SMTP Virtual server in IIS
      • General Settings
        • You will see an IP in the middle, this is the IP of the Relay server itself.
        • Select Advanced next to the Relay Server IP
    • Select Add
      • Choose the IP of the VIP created earlier
      • Set the port to 25
    • Access Settings
      • Select Relay

    Select Add


    Add the VIP you created earlier

    • IIS SMTP Relay is now configured

    Exchange SMTP Relay Configuration

    • Open Exchange Management Console
    • Expand Server Configuration
      • Select Hub Transport
        • Select a Hub Transport
    • In the “Use these local IP addresses to receive mail” section, select Add
      • Select “Specify an IP address”
      • Enter the VIP you created earlier
      • Set the Port to 25
    • You now need to do the same step to all of your Hub Transport Servers
    • Your Exchange SMTP Relay is now Configured

    Read this if DSR and Netscaler isn't working for you


    As per the comment from rbarrick on the above article, I also found the same problem. I had more than one VIP with the same IP and even though they were disabled it would not connect until I deleted the disabled Virtual Servers with the same IP. I had them setup after following this article on Exchange Load balancing so when it came to configuring DSR I had multiple VIP's with the same IP. Deleting them or changing the IP they use is the only solution. Hope this saves someone some head scratching.

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