This article describes the issue of Microsoft NLB's cluster IP address being unreachable from clients connected in the same VLAN, when configured in IGMP-multicast. mode.

• Network Load Balancing is a clustering technology offered by Microsoft, as part of all Windows 2000 Server and Windows Server 2003 family operating systems.


• NLB uses a distributed algorithm to load balance network traffic across a number of hosts, helping to enhance the scalability and availability of mission critical IP-based services, such as Web, Virtual Private Networking, Streaming Media, Terminal Services, Proxy, and so on.


• It also provides high availability by detecting host failures and automatically redistributing traffic to operational hosts.


• A cluster is group of connected independent computers, using layer 2 or layer 3 switches. When implementing Microsoft NLB and servers are connected to EX switches, special care needs to be taken when configuring EX switches.

When Microsoft NLB servers and clients are connected to the EX switch in the same VLAN, the EX switch will not be able to forward IP packets to Microsoft NLB's cluster IP address, if the RVI (Routed Virtual Interface) interface is configured for the same VLAN.

The solution is applicable to Junos Code 11.4 and later.

Microsoft NLB, when setup in the IGMP-multicast mode, uses the unicast cluster IP address; but associates multicast MAC addresses [01:00:5e:00:00:00 - 01:00:5e:7f:ff:ff] with it. Any IP packets to cluster IP address should be forwarded to multicast MAC address.

One of the following options can be applied to resolve the issue:

• Disable the RVI (Routed VLAN Interface) for the NLB server VLAN. 


• Disable IGMP-snooping on EX switch using the following command:
  
  

  set protocol igmp-snooping vlan <name of the vlan or All> disable



• If IGMP snooping cannot be disabled for the server VLAN, configure the multicast-router-interface for all the NLB server interfaces connected to the switch. The following command can be used to make the NLB server interface a multicast-router-interface; If the server interface is ge-5/0/0:
  
  

  Set protocols igmp-snooping vlan vlan-20 interface ge-5/0/0.0 multicast-router-interface

Note: Prior to the 11.4 release, the EX switch would drop any packets with multicast destination MAC and unicast destination IP, if the RVI is configured in that VLAN; regardless of the IGMP-snooping configuration.