Tag Archives: virtual

Hyper-V VM loses network connectivity intermittently

I just resolved an issue where a Hyper-V virtual machine was running fine for a few weeks, then it suddenly dropped off the network and connectivity went unpredictably intermittent.

Couldn’t remote desktop to it, pings drop most of the time but not all of the time; they looked like this:

Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.237: bytes=32 time=1098ms TTL=126
Reply from 172.24.255.237: bytes=32 time=1ms TTL=126
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.237: bytes=32 time<1ms TTL=126
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.153: Destination host unreachable.
Reply from 172.24.255.237: bytes=32 time<1ms TTL=126
Reply from 172.24.255.237: bytes=32 time<1ms TTL=126

Live replies from the correct address, unreachable response from a different address (but another Hyper-V virtual machine).

The Hyper-V environment is composed of five Windows Server 2012 R2 Standard server loaded on top of a five-blade Cisco UCS B200 M3 with Nimble SAN.

Google found lots of wrong answers involving disabling VMQ on the host and guest, but my new hero Joel Coel mentioned some of his Hyper-V guests had been given duplicate MAC addresses.

Sure enough, I checked the guests with those two IP addresses and they had the same MAC:

These two Hyper-V guests have the same MAC address.

I solved the conflict by turning off the VM, removing the Network Adapter with the duplicate MAC, Applying the change, then adding a new NIC.

Technet’s Gilson Banin wrote how  to solve the root cause – Hyper-V servers with duplicate MAC pools.