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question on cluster private and public network Expand / Collapse
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Posted Wednesday, July 23, 2014 1:57 PM
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Hi,

On a cluster, we have 2 networks i.e private network and public network.
Private network is used for cluster communication(LooksAlive and IsAlive checks) and Public network is used for client applications to connect.

Now my question is, what happens if Private network is down. Does it have any effect on cluster? or Does it failback to Public network?
I have only theoretical knowledge and I haven't worked on clusters. Just wanted to know what happens when private network goes down?

Thanks in advance.
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Posted Thursday, July 24, 2014 4:04 AM


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vsamantha35 (7/23/2014)
Hi,

On a cluster, we have 2 networks i.e private network and public network.
Private network is used for cluster communication(LooksAlive and IsAlive checks) and Public network is used for client applications to connect.

Now my question is, what happens if Private network is down. Does it have any effect on cluster? or Does it failback to Public network?
I have only theoretical knowledge and I haven't worked on clusters. Just wanted to know what happens when private network goes down?

Thanks in advance.

When the private network goes down cluster communications will carry on through the public network. Loss of a private network does not initiate a failover. The loss of the public network will initiate a failover since this is the network clients connect through.


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Post #1595837
Posted Saturday, August 2, 2014 2:26 PM
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When the private network goes down cluster communications will carry on through the public network. Loss of a private network does not initiate a failover. The loss of the public network will initiate a failover since this is the network clients connect through.


Hi Perry,

Thanks for the reply. Just a question on top of it to make sure whether I understood correctly or not.
Now say, If I have a 2 node cluster (N1 and N2), and I have 2 dedicated NICs, one for only private network and one for only public network only. quorum model is Node+Disk witness as it is even number of nodes.

Assume, N1 is am active node and now if private network fails on N1, then what happens?
I am assuming that since N1 is not able to communicate with N2, it loses the cluster membership, cluster service is stopped on N1 and a fail-over is initiated to N2 and before that Quorum is recalculated and if I have 2 votes then I would fail-over to N2. Is that correct?

Pl correct me if am wrong Perry.

Thank you.
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Posted Tuesday, August 5, 2014 3:49 AM This worked for the OP Answer marked as solution


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vsamantha35 (8/2/2014)
When the private network goes down cluster communications will carry on through the public network. Loss of a private network does not initiate a failover. The loss of the public network will initiate a failover since this is the network clients connect through.


Hi Perry,

Thanks for the reply. Just a question on top of it to make sure whether I understood correctly or not.
Now say, If I have a 2 node cluster (N1 and N2), and I have 2 dedicated NICs, one for only private network and one for only public network only. quorum model is Node+Disk witness as it is even number of nodes.

Assume, N1 is am active node and now if private network fails on N1, then what happens?

The cluster virtual network adapters on each node would communicate over the next availble network pipe, in this case the Public network.



vsamantha35 (8/2/2014)
[quote]I am assuming that since N1 is not able to communicate with N2, it loses the cluster membership, cluster service is stopped on N1 and a fail-over is initiated to N2 and before that Quorum is recalculated and if I have 2 votes then I would fail-over to N2. Is that correct?

Pl correct me if am wrong Perry.

Thank you.

No, that is not the case. A failure in a secondary network will not cause a failover, a failure of the primary\public network will initiate a failover


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Post #1599537
Posted Tuesday, August 5, 2014 11:25 PM
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Thanks Perry. Again, what is split brain? I have read that whenever private/heartbeat network fails, split brain occurs?
Consider I have a 3 node cluster (N1,N2,N3) and private network completely is down on N1 and cannot communicate to other nodes.
and private network is not set for "All communications", so I would a expect a failover from N1 to next possible owner. It could be N2 or N3 depending on install order.

1) Now, my doubt is when does the split brain happen or when can a split brain situation occurs?
2) What happens if a split brain occur?
3) What will happen to a cluster in that scenario? Is there any auto healing functionality to come out of that situation?
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Posted Wednesday, August 6, 2014 5:26 AM This worked for the OP Answer marked as solution


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vsamantha35 (8/5/2014)
Thanks Perry. Again, what is split brain? I have read that whenever private/heartbeat network fails, split brain occurs?

It happens less and less with the latest versions of Windows Server due to enhancements within the WSFC subsystem. The split brain scenario occurs when subsets of the cluster nodes are separated or partitioned by a network outage, say between sites. Both sets of nodes may then attempt to own the same shared resources causing corruption in extreme cases.


vsamantha35 (8/5/2014)
Consider I have a 3 node cluster (N1,N2,N3) and private network completely is down on N1 and cannot communicate to other nodes.
and private network is not set for "All communications", so I would a expect a failover from N1 to next possible owner. It could be N2 or N3 depending on install order.

1) Now, my doubt is when does the split brain happen or when can a split brain situation occurs?
2) What happens if a split brain occur?
3) What will happen to a cluster in that scenario? Is there any auto healing functionality to come out of that situation?

In Windows 2008 onwards you do not need a dedicated private\heartbeat network. You do need multiple redundant networks but they’re not strictly reserved for heartbeat communications. The cluster nodes each have a virtual cluster network adapter that binds itself to an available cluster network that is set to allow cluster communications. The public network has this by default.

A cluster network has 3 possible settings

  • allow cluster comms

  • allow cluster comms and client connections

  • do not allow cluster comms


The split brain occurs when all network paths are broken and subsets of nodes cannot communicate with a partitioned subset. Again, a failure of a heartbeat network only does not initiate a failover as cluster communication will continue over the next available network enabled for cluster communication.


Build yourself a virtual test cluster and test this yourself easily.


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Post #1600154
Posted Wednesday, August 6, 2014 7:38 AM
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Build yourself a virtual test cluster and test this yourself easily


This advice is good. You can learn a lot from books and forums, but these will not let you experience the horror of a broken cluster and the satisfaction of (hopefully) fixing it.

You really need to put a cluster together in a virtual environment to work out exactly what is needed in your own organisation.


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Post #1600229
Posted Wednesday, August 6, 2014 1:08 PM
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Thanks a lot Perry. so kind of you. I am not a expert on cluster. Just want to clear some concepts and definitely I will try building VMs and try some of this stuff. Again, thank you very very much. Appreciate it.
Post #1600362
Posted Wednesday, August 6, 2014 3:10 PM


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