Q: Why, on my Windows Server 2012 R2 RTM installation, is the Microsoft Failover Cluster Virtual Adapter Performance Filter disabled?

A: Nodes in a cluster communicate with each other via a cluster virtual network adapter, NetFT, which is actually visible if you run IPCONFIG /AL--for example:

Tunnel adapter Local Area Connection* 11:

Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft Failover Cluster Virtual Adapter
Physical Address. . . . . . . . . : 02-48-4D-56-12-79
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : fe80::cc19:27fd:67:73f4%23(Preferred)
IPv4 Address. . . . . . . . . . . : 169.254.2.86(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.0.0
Default Gateway . . . . . . . . . :
DHCPv6 IAID . . . . . . . . . . . : 688049663
DHCPv6 Client DUID. . . . . . . . : 00-01-00-01-19-B8-19-EC-00-26-B9-43-DA-12

NetBIOS over Tcpip. . . . . . . . : Enabled

The NetFT virtual adapter works by implementing its own type of NIC teaming that can utilize multiple potential paths across the physical network adapters that allow network communications. The communications therefore initially travel over UDP to the NetFT virtual adapter, then back through the network stack to a physical network adapter.

What the Microsoft Failover Cluster Virtual Adapter Performance Filter (introduced in Windows Server 2012), does is when it sees any cluster traffic on the physical network adapter, it sends it to NetFT adapter directly, bypassing the redirection through physical network stack.

This sounds good. However, if you also have a guest cluster running on virtual machines (VMs) within the Hyper-V host cluster, and guest VMs are running on different nodes in the cluster, the performance filter would grab not only the host cluster communications but also the guest cluster communications. This means the communication would never reach the VMs and therefore the clusering would break.

To resolve this problem, the Microsoft Failover Cluster Virtual Adapter Performance Filter would need to be disabled in Windows Server 2012, which is why it's disabled by default on Windows Server 2012 R2.