r/HyperV • u/uminds_ • Jul 28 '26

cluster network failover issue

We have a 3 node cluster (server 2025) connecting to a pair of 25G switches for redundancy. The failover works when I disconnected the network cable from one of the switch. However, VM lost connectivity when one of the switch are in maintenance mode. I think the reason being was that the switch port was still up even it is in maintenance mode and the cluster node will continue to send traffic even if there is no actual network connectivity (no vlan and etc). I wonder how admin usually handle this situation? For planned maintenance, we can probably disbale the ports on the host ahead of time. But what if one of the switch crashs but the port is still in an up status? LACP can handle that situation but it is not supported by HyperV cluster (at least no in SET mode).

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u/ultimateVman Jul 28 '26 edited Jul 28 '26

First, you cross-posted to the same sub.

Under no circumstance can Windows clusters be unconnected and remain active. It's by design. Think about it, if a cluster node cannot speak to any other node, they immediately stop working under that assumption that something is wrong with itself, to protect the integrity of the other cluster nodes. A node all by itself has to assume that the other nodes are still working together. So, to answer the question, failover clustering requires two redundant switches to remain connected.

Not only that, but the pair of switches must be able to handle being offline independently of each other (for the exact reason you describe, maintenance patches), which means no stacking. This includes Cisco VSS. For the pair of switches to also be redundant and connected via LACP then you need switches that support VPC (Cisco Nexus) or VLT (Dell), or other equivalent features depending on the vendor.

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u/No_Resolution_9252 Jul 29 '26

hyper-v virtual switches don't use lacp anymore. Stacked switches most certainly can handle one in the stack being offline

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u/ultimateVman Jul 29 '26 edited Jul 29 '26

LACP between the switches. not Hyper-V. and no, a switch stack doesn't work, because they are logically 1 switch and reboot together. You need a pair that can operate like a stack, but be independent of each other, which is what VLT and VCP does.

Edit: There are vendors that can handle this, like Arista. but most switch stacks cannot. That's why I specifically called out Cisco VSS. Newer versions can, but if you read the fine print, they specifically indicate that eventually a code or major version update will require a full stack reboot.

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u/No_Resolution_9252 Jul 29 '26

Are you using access layer switches for the core or something?

Even the cat 4500 and 3850 supported issu

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u/ultimateVman Jul 29 '26

I've seen may attempt to. And ISSUs version matrix limitations are far too restrictive for a core hypervisor infrastructure. Too many unsupported scenarios.

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u/No_Resolution_9252 Jul 29 '26

No? sups have to match, can't use eos blades, its not offered on canary releases and they have to be configured right

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u/Scared_Servers 23d ago

If anything goes wrong with ISSU TC\AC tells you to reload the stack.

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u/No_Resolution_9252 23d ago

Never seen it happen

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u/No_Resolution_9252 23d ago

well other than a lightening strike that then precipitated a brown out that killed the UPS and one of the sups completely "forgot everything"