GavinW70
Contributor
Contributor

Vswitch sending ARP Packet back from Source

We created a standard ESXi vswitch with a single uplink enabled, configured in promiscuous mode, and attached to a VM deployed inside the ESXi. VMs inside the ESXI are deployed in bridge mode (L2).

We have seen that when system is loaded with traffic the ARP request coming from the VMs sides (bridge mode) and going out through the ESXi vswitch are sent back by the vswitch through the same port where it comes from which is not a typical HUB or switch behavior. On the VM interface connected to the vswitch we see that the ARP request get out of the interface toward the vswitch and then comeback through the same interface within milliseconds.

It was well verified through troubleshooting that:

  • these send back from same interface ARP are coming from the ESXi vswitch
  • the issue was seen on different ESXi versions 6.5, 6.7 and 7.0
  • when having a vswitch without an uplink configured and traffic doesn't communicate to the ESXi outside world we don't see the issue
  • issue comes only when system is under traffic load (in our case we were sending 100Mbps)

To overcome this issue, we had to deploy a linux VM that have interface passthrough enabled and that will act as a vswitch to prevent using the ESXi vswitch. This work around is costly because it will dedicate the physical interface to that VM and we no longer can use that for other VMs. Has anyone else seen this, or is this something VMWAre can investigate for us, as we see this as a Bug.

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8 Replies
Perttu
Enthusiast
Enthusiast

Can you confirm that this ARP packet is not seen twice on the uplink as well, first on egress and then on ingress again. Within milliseconds sounds to me that the packet makes a longer trip than just the vSwitch before looping back. Otherwise it should be a matter of sub-milliseconds. 

Also this suggest that the issue could be elsewhere

  • when having a vswitch without an uplink configured and traffic doesn't communicate to the ESXi outside world we don't see the issue
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scott28tt
VMware Employee
VMware Employee

@GavinW70 

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GavinW70
Contributor
Contributor

I can confirm that while troubleshooting this we only saw one ARP request exiting the ESXi and one reply is back, ARP packet is not seen twice on the uplink within that time frame. Aslo on original Question I was incorrect I was saying Millisecond, however it was suppose to say Microsecond.

 

Please see below capture showing the timing

 

ARP request getting out from VM toward the vswitch:
12:01:04.087056 00:90:0b:66:5d:29 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 64: vlan 101, p 0, ethertype ARP, Request who-has 192.168.101.1 tell 192.168.101.2, length 46
ARP request coming in at the VM from vswicth:
12:01:04.087126 00:90:0b:66:5d:29 > ff:ff:ff:ff:ff:ff, ethertype 802.1Q (0x8100), length 64: vlan 101, p 0, ethertype ARP, Request who-has 192.168.101.1 tell 192.168.101.2, length 46
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GavinW70
Contributor
Contributor

The difference is 70 Micro seconds

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Perttu
Enthusiast
Enthusiast

Interesting.

Can you capture the rebound packet on the ESXi (not only on the VM) as well. Here's a spell for that: 

pktcap-uw --switchport <port_ID> --dir 2 --ethtype 0x0806

This seems to be a bug, but is it related explicitly to promiscuous mode? If I recall correctly promiscuous mode also had some performance penalty. You seem to talk about VSS, but are you eligible for VDS and could you try this instead?

https://www.virtuallyghetto.com/2018/04/native-mac-learning-in-vsphere-6-7-removes-the-need-for-prom...

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GavinW70
Contributor
Contributor

attached is the capture for the arp packets getting out to the VM at the vswitch portgoup level.
it shows exactly what we need, the arp rebound packet is seen and then we see the server arp reply
 
 
 

 

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GavinW70
Contributor
Contributor

Thank you for your advise, can you tell me how I can find out if we are eligible for VDS, is this something we have to pay for?

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Perttu
Enthusiast
Enthusiast

I'm not that aware of VMware licenses and about what options are included in which license edition and so forth, but I believe that VDS (Virtual Distributed Switch) needs least a vCenter license and hence few paid vSphere licenses for hosts as well. Hence this is not possible with free edition.

 

   

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