Microsoft NLB IP issue

sree hari 0 Reputation points


I have 2 Windows server 2019 VMs. We have configured NLB cluster in multicast mode and added both the VMs. The Cluster IP address is not reachable to other VMs. However, the host IPs (both vm IPs) are reachable to others. All IPs are in same subnet. There are no Firewall devices in between.

Windows Server 2019
Windows Server 2019
A Microsoft server operating system that supports enterprise-level management updated to data storage.
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  1. Jing Zhou 5,210 Reputation points Microsoft Vendor



    Thank you for posting in Q&A forum.

    Based on the situation you described, this situation may be caused by NLB configuration issues in multicast mode. In the multicast mode of NLB, there may be some network communication challenges, especially in virtualized environments. Here are some common reasons and solutions that may cause cluster IP unreachable:

    ARP caching issue: In multicast mode, it may cause incorrect ARP caching, making it difficult for other devices to correctly resolve the MAC address corresponding to the cluster IP address. You can try clearing the ARP cache of the relevant devices or manually adding ARP entries to solve this problem.

    Network device configuration: Network device configuration in a virtualized environment may also affect the normal operation of NLB. Please ensure that the network switch or router is properly configured to support NLB multicast communication.

    NLB parameter settings: Check the parameter settings of the NLB cluster to ensure that the cluster IP address, port rules, etc. are correctly configured. Sometimes it may be necessary to adjust the parameters of NLB to adapt to specific network environments.

    Network traffic routing: Ensure that network traffic is correctly routed to the NLB cluster, and that the traffic returned from the cluster can also be correctly returned to the requesting device. It may be necessary to check the routing table and network device configuration.

    If you can provide more detailed information about NLB configuration, such as the specific settings of NLB clusters, virtualization platforms, etc., I can provide you with more specific help and suggestions. I hope this information is helpful to you!


    Best regards,

    Jill Zhou



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  2. sree hari 0 Reputation points


    Thanks for your reply.

    We are running Windows server 2019 standard on VMware vSphere 7 with distributed switch. We have the VM connectivity to Cisco business series 350 switch. The NLB is configured with Multicast mode and ports are on default configuration.

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