The NIC failover does not work in LBT mode

Dear community,

We met a strange behavior in our network environment while testing our new Cisco 4500 switches x 10g. If you have a TCP Session (example JJ | netcat) data transfer between two virtual machines on different host esx failover does not work on the VM receiver if a physical connection fails.

The test configuration is as follows:

|    | - 4 - switch 1 vmnic - vmnic 4-|    |

VM1 | ESX1 |                  | VSS |                    | ESX2 |-VM2

|    | - 5 - switch 2 vmnic - vmnic 5-|    |

More details:

  • The switches are configured as a cluster VSS. New version of the software
  • ESX1/2 are running the latest version 5.5
  • The two vmnic is active on a DV-switch in LBT mode
  • Exchanges are configured with "Port based on the physical NIC load."
  • VM1 and VM2 are invited linux, NICS are vmxnet3

Steps to reproduce:

  1. Beginning listening mode on VM2 (netcat-l Pei 12345 |) JJ = / dev/null)
  2. Start the transfer on VM1 (dd if = / dev/zero bs = 1 M | netcat VM2 12345)
  3. Check which link is used (in my example vmnic5 on ESX2)
  4. Unplug the ESX2 vmnic5

What you observe:

  • DD-traffic dies immediately
  • If you kill the dd on machines (it does not end by itself) and start all over again, it works immediately
  • If you ping VM1 VM2 or another system, it works
  • If you plugged vmnic5 again once the virtual machine is switched to new vmnic5-> if you didn't kill dd will continue

Now to test the reverse:

  1. Beginning listening mode on VM2 (netcat-l Pei 12345 |) JJ = / dev/null)
  2. Start the transfer on VM1 (dd if = / dev/zero bs = 1 M | netcat VM2 12345)
  3. Check which link is used (in my example vmnic4 on ESX1)
  4. Disconnect vmnic1 ESX1

What you observe:

  • Failover works like a charm
  • Failback works like a charm


  • Only the virtual machine at the front desk has a problem
  • Only an already established TCP session behaves badly, work of new sessions very well (as does ping, no losses)
  • If you use LACP place LBT on uplink the problem disappeared
  • Same problem if you connect two links to Switch1
  • Same problem if you use a physical box with linux and LACP as system
  • I did the same test with two c3750x (no cluster VSS) and has not encountered the problem

Any ideas? It's really a good one.

Thank you!



To make it clear, it doesn't have the transmission of UDP.

Post edited by: sewert1


I did some further testing and also involved cisco. There is no problem with the switches and network in general.

Turns out he works with Windows clients, but it does not work with Linux. But it works with Linux with adapters e1000 and e1000e but not with vmxnet3.

So I did some tests and learned that the vmxnet3 kernel module must be at least version 1.3. Version 1.1 (SLES 11 default) or 1.2 (default SLES12) does not work.

(./ = vmxnet3)

Tags: VMware

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