Server 2019 Windows NLB - failover individual ports?

Blaze92 1 Reputation point

Hi all,

We have two servers, a Primary and a Secondary, which work in a failover scenario. We run on the primary and the secondary is ready to take over in the event of a failure.

We have other servers which send traffic to these servers. Some of them are only capable of sending traffic to a single IP address, so to avoid manual configuration in a failover event, we have configured NLB to send this traffic to a single address. This is configured in Single Host mode where primary has priority 1 and secondary has priority 2.

My question is, when traffic is sent to the NLB cluster address, will it look at the individual port on the primary server to see if it is open and if not, send it to the secondary, or will it only look to see if the primary is online.

Eg if we had multiple ports, say 8000 and 9000, that we were sending data on, and the primary stopped listening on 8000, but was still listening on port 9000, should it direct the traffic on 8000 to the secondary while the traffic on 9000 continued to go to the primary?

Or another example, lets say the server stopped listening on all ports because the services stopped, but the server itself was still running so the NLB address was still available, would it just try to send the traffic to the Primary as it still thinks it is online, even though it is not listening on those ports?

We had this set up on some Server 2012 R2 servers and I am sure this was tested and working where traffic would go to the secondary if the primary was not able to receive it, even if it was online. However we have just upgraded to new Server 2019 servers and I am finding that the NLB is only failing over to the secondary if I actually disable the NLB interface on the primary so the NLB cluster sees it as offline. It is possible I am mistaken as to how it was working on the old servers, as it is very rare that we need to failover.

Is this how it is supposed to work? Or should traffic go to the first server that can receive it, not just the first server that is online in the cluster?

Windows Server Clustering
Windows Server Clustering
Windows Server: A family of Microsoft server operating systems that support enterprise-level management, data storage, applications, and communications.Clustering: The grouping of multiple servers in a way that allows them to appear to be a single unit to client computers on a network. Clustering is a means of increasing network capacity, providing live backup in case one of the servers fails, and improving data security.
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  1. Limitless Technology 39,501 Reputation points

    Hello @Blaze92 ,

    When you deploy VMs as virtual clusters, NLB does not require servers to be multi-homed to have multiple virtual IP addresses.

    The Network Load Balancing (NLB) feature distributes traffic across several servers by using the TCP/IP networking protocol.

    To provide high availability, NLB includes built-in features that can automatically:

    Detect a cluster host that fails or goes offline, and then recover.

    Balance the network load when hosts are added or removed.

    Recover and redistribute the workload within ten seconds.

    For further information do follow the below link


    Hope this answers all your queries, if not please do repost back.
    If an Answer is helpful, please click "Accept Answer" and upvote it : )

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