Recently I am investigating NLB VIP unstable issue. I also found another issue with ARP, it's might be same issue with NLB.
environment info please see this thread: https://social.technet.microsoft.com/Forums/en-US/df4722a9-62fa-4d34-a1d3-9dd2de0fa8cd/hyperv-nlb-vip-unable-to-ping-from-some-clients?forum=winserverClustering
I found when NLB VIP get unstable, some VM nodes within this NLB cannot ping it's GW of NLB dedicate NIC. but the LAN NIC ping gateway is pretty fine.
After configure port mirror on TOR switch, I can capture ARP reply package from Cisco Switch. but on Hyper-V host, I can only see ARP package send out, but without any ARP response inbound.
Reboot V-Host or just disable then enable NIC on Hyper-V host, can fix this issue temporary. but after hours or one day, this issue will repro again.
by netsh trace log, found two strange reason. all VM have been disabled MAC Address spoofing and never use NIC bridge.
[0]0000.0000::2017-01-18 14:08:42.090 [Microsoft-Windows-Hyper-V-VmSwitch]NBL destined to Nic 5ABC7209-83A0-4034-A30C-1D34B066AC9C--71761E64-BF59-4A0D-93CC-3F7C25021AEF (Friendly Name: Network Adapter) was dropped in switch 037C29CB-B632-498B-B1B3-D676884AF17D
(Friendly Name: VLAN_693_10.185.76.96_27), Reason Bridge is not allowed to run inside VM
[24]0004.29E8::2017-01-18 12:47:07.651 [Microsoft-Windows-Hyper-V-VmSwitch]NBL originating from Nic 0C493B45-B40B-46BC-BD2D-835A95D7C19A--B56F8597-A216-406E-B1C3-187246E51EA4 (Friendly Name: Network Adapter) was dropped in switch 766ED074-5690-4A47-9819-8CF160B99D7E
(Friendly Name: VLAN_692_10.185.76.64_27), Reason Spoofed MAC address is not allowed
