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EX Series Switches and Microsoft Network Load Balancing (NLB) in multicast mode



Article ID: KB14854 KB Last Updated: 21 Sep 2015Version: 2.0

Network Load Balancing (NLB) 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, etc. It also provides high availability by detecting host failures and automatically redistributing traffic to operational hosts.

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


Microsoft NLB's cluster IP address is not reachable from clients connected in the same VLAN when connected to the EX switch.


Microsoft Network Load Balancing (NLB) can be configured in three modes:

1. Unicast
2. Multicast
3. Multicast with IGMP

Microsoft NLB, when set up in multicast or multicast IGMP mode, uses a unicast cluster IP address but associates a multicast MAC address with it. Any IP packets sent to the cluster IP address will be forwarded to the multicast MAC address.

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 Microsft NLB's cluster IP address if the RVI (Routed Virtual Interface) interface is configured for the same VLAN.  This has been fixed in the Junos OS software with PR434505 (see Related Links, below).

If the Microsoft NLB is configured in multicast or multicast IGMP mode, it sends a multicast IGMP V1/V2 report to the switch for multicast group address 239.255.x.x. If the EX switch is configured for IGMP snooping or IGMP snooping is not disabled (IGMP snooping is enabled by default in EX after the 9.3X release) packets flooding to the multicast MAC address does not happen, which prevents packet forwarding to Microsoft NLB's cluster IP address. In this case, IGMP snooping needs to be disabled on the EX switch using the command shown below:

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

For example:

set interfaces vlan.1 family inet address arp multicast-mac 01:00:5e:00:00:04

As per the documentation from Microsoft and Layer 3 default behavior of the switch, you have to add a static ARP entry in the switch for the cluster IP address (unicast IP to multicast MAC) if it is to be reached from another IP subnet.

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