Configure Network Load Balancing(NLB) Multicast Mode

How to Configure Network Load Balancing (NLB) in Multicast Mode

Understanding Multicast Mode

Multicast mode does not have the problem that unicast operation does since the servers can communicate with each other via the original addresses of their NLB network cards.

Each server’s NLB network card operating in multicast mode has two MAC addresses (the original one and the virtual one for the cluster), which causes some problems. Most routers reject the ARP replies sent by hosts in the cluster, since the router sees the response to the ARP request that contains a unicast IP address with a multicast MAC address. The router considers this to be invalid and rejects the update to the ARP table. In this case, you need to manually Configure Static ARP Resolution at the switch or router for each port connecting to ESX’s NICs.

NLB Multicast Mode – Static ARP Resolution

  • Since NLB packets are unconventional, meaning the IP address is Unicast while the MAC address of it is Multicast, switches and routers drop NLB packets.
  • NLB Multicast Packets get dropped by routers and switches, causing the ARP tables of switchesto not get populated with cluster IP and MAC address.
  • Manual ARP Resolution of NLB cluster address is required on physical switch and router interfaces
  • Cluster IP and MAC static resolution is set on each switch port that connects to ESX/ESXi host
  • Virtual Switch NIC Team Policy > Notify Switches is set to Yes.
  • If CDP (Cisco Discovery Protocol) is Enabled on ESX/ESXi and the Cisco Switch – you can determine proper switch ports connecting to ESX/ESXi via the vSphere/Virtual Infrastructure Client.

A sample Configuration of Static ARP on CISCO Switch:

To configure the switch:

  1. Telnet in to Cisco Switch Console and log in.
  2. Run this command to enter Configuration mode:config t
  3. STATIC ARP RESOLUTION Cisco Global command mode For example:arp [ip] [cluster multicast mac] ARPA arp 03bf.c0a8.0164 ARPA
  4. STATIC MAC RESOLUTION Cisco Global command modeFor example:mac-address-table static [cluster multicast mac] [vlan id] [interface] mac-address-table static 03bf.c0a8.0164vlan1 interface GigabitEthernet1/1 GigabitEthernet1/2 GigabitEthernet1/15 GigabitEthernet1/16

Setting up NLB Multicast Mode on VMware node:

Sample Setup

These versions of Windows are recommended:

  • Microsoft Windows 2008 Server
  • Microsoft Windows 2008 R2
  • Microsoft Windows 2003 Server and later
  • Microsoft Windows 2000 Server with Load Balancing

To configure NLB in Windows Server 2003:

Note: Each NLB cluster node is required to have NLB enabled and configured with the same cluster IP address and FQDN.

  1. Go to Local Area Connection Properties >General tab > Check Network Load Balancing.
  2. Click Properties.
  3. Click the Cluster Parameters
  4. Enter the Cluster IP address.
  5. Select the MulticastNote: Click to enlarge image.
  6. Click the HostParameters tab
  7. Set the Priority (unique host identifier).
  8. Enter the dedicated host IP. This is the same as the NIC IP. (From the example above this would be for the first host)
  9. Click OK.Note: Click to enlarge image.
  10. Select Networking Protocols TCP/IP
  11. Enter the dedicated host IP specified in Step 8.
  12. Click Advanced.
  13. Click Add.
  14. Enter the Cluster IP.
  15. Click OK.

Note: Click to enlarge image.

For additional information, see Sample Configuration – Network Load Balancing (NLB) Multicast mode over routed subnet – Cisco Switch Static ARP Configuration (1006525).

For more information on weak and strong host behavior in Windows, see the Microsoft Technet article

For more information on configuring Network Load Balancing (NLB) on Windows Server 2008, see the Microsoft Technet article

Some more links.